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## Chapter 11. HQL and JPQL

The Hibernate Query Language (HQL) and Java Persistence Query Language (JPQL) are both object model focused query languages similar in nature to SQL. JPQL is a heavily-inspired-by subset of HQL. A JPQL query is always a valid HQL query, the reverse is not true however.

Both HQL and JPQL are non-type-safe ways to perform query operations. Criteria queries offer a type-safe approach to querying. See ??? for more information.

## 11.1. Case Sensitivity

With the exception of names of Java classes and properties, queries are case-insensitive. So `SeLeCT` is the same as `sELEct` is the same as `SELECT`, but `org.hibernate.eg.FOO` and `org.hibernate.eg.Foo` are different, as are `foo.barSet` and `foo.BARSET`.

## Note

This documentation uses lowercase keywords as convention in examples.

## 11.2. Statement types

Both HQL and JPQL allow `SELECT`, `UPDATE` and `DELETE` statements to be performed. HQL additionally allows `INSERT` statements, in a form similar to a SQL `INSERT-SELECT`.

## Important

Care should be taken as to when a `UPDATE` or `DELETE` statement is executed.

| Caution should be used when executing bulk update or delete operations because they may result in inconsistencies between the database and the entities in the active persistence context. In general, bulk update and delete operations should only be performed within a transaction in a new persistence con- text or before fetching or accessing entities whose state might be affected by such operations. |
| --Section 4.10 of the JPA 2.0 Specification

### 11.2.1. Select statements

The BNF for `SELECT` statements in HQL is:

    select_statement :: =
            [select_clause]
            from_clause
            [where_clause]
            [groupby_clause]
            [having_clause]
            [orderby_clause]

The simplest possible HQL `SELECT` statement is of the form:

    from com.acme.Cat

The select statement in JPQL is exactly the same as for HQL except that JPQL requires a `select_clause`, whereas HQL does not. Even though HQL does not require the presence of a `select_clause`, it is generally good practice to include one. For simple queries the intent is clear and so the intended result of the `select_clause` is east to infer. But on more complex queries that is not always the case. It is usually better to explicitly specify intent. Hibernate does not actually enforce that a `select_clause` be present even when parsing JPQL queries, however applications interested in JPA portability should take heed of this.

### 11.2.2. Update statements

The BNF for `UPDATE` statements is the same in HQL and JPQL:

    update_statement ::= update_clause [where_clause]

    update_clause ::= UPDATE entity_name [[AS] identification_variable]
            SET update_item {, update_item}*

    update_item ::= [identification_variable.]{state_field | single_valued_object_field}
            = new_value

    new_value ::= scalar_expression |
                    simple_entity_expression |
                    NULL

`UPDATE` statements, by default, do not effect the `version` or the `timestamp` attribute values for the affected entities. However, you can force Hibernate to set the `version` or `timestamp` attribute values through the use of a `versioned update`. This is achieved by adding the `VERSIONED` keyword after the `UPDATE` keyword. Note, however, that this is a Hibernate specific feature and will not work in a portable manner. Custom version types, `org.hibernate.usertype.UserVersionType`, are not allowed in conjunction with a `update versioned` statement.

An `UPDATE` statement is executed using the `executeUpdate` of either `org.hibernate.Query` or `javax.persistence.Query`. The method is named for those familiar with the JDBC `executeUpdate` on `java.sql.PreparedStatement`. The `int` value returned by the `executeUpdate()` method indicates the number of entities effected by the operation. This may or may not correlate to the number of rows effected in the database. An HQL bulk operation might result in multiple actual SQL statements being executed (for joined-subclass, for example). The returned number indicates the number of actual entities affected by the statement. Using a JOINED inheritance hierarchy, a delete against one of the subclasses may actually result in deletes against not just the table to which that subclass is mapped, but also the "root" table and tables “in between”

Example 11.1. Example UPDATE query statements

    String hqlUpdate =
    		"update Customer c " +
    		"set c.name = :newName " +
    		"where c.name = :oldName";
    int updatedEntities = session.createQuery( hqlUpdate )
            .setString( "newName", newName )
            .setString( "oldName", oldName )
            .executeUpdate();

    String jpqlUpdate =
    		"update Customer c " +
    		"set c.name = :newName " +
    		"where c.name = :oldName";
    int updatedEntities = entityManager.createQuery( jpqlUpdate )
            .setString( "newName", newName )
            .setString( "oldName", oldName )
            .executeUpdate();

    String hqlVersionedUpdate =
    		"update versioned Customer c " +
    		"set c.name = :newName " +
    		"where c.name = :oldName";
    int updatedEntities = s.createQuery( hqlUpdate )
            .setString( "newName", newName )
            .setString( "oldName", oldName )
            .executeUpdate();

## Important

Neither `UPDATE` nor `DELETE` statements are allowed to result in what is called an implicit join. Their form already disallows explicit joins.

### 11.2.3. Delete statements

The BNF for `DELETE` statements is the same in HQL and JPQL:

    delete_statement ::= delete_clause [where_clause]

    delete_clause ::= DELETE FROM entity_name [[AS] identification_variable]

A `DELETE` statement is also executed using the `executeUpdate` method of either `org.hibernate.Query` or `javax.persistence.Query`.

### 11.2.4. Insert statements

HQL adds the ability to define `INSERT` statements as well. There is no JPQL equivalent to this. The BNF for an HQL `INSERT` statement is:

    insert_statement ::= insert_clause select_statement

    insert_clause ::= INSERT INTO entity_name (attribute_list)

    attribute_list ::= state_field[, state_field ]*

The `attribute_list` is analogous to the `column specification` in the SQL `INSERT` statement. For entities involved in mapped inheritance, only attributes directly defined on the named entity can be used in the `attribute_list`. Superclass properties are not allowed and subclass properties do not make sense. In other words, `INSERT` statements are inherently non-polymorphic.

`select_statement` can be any valid HQL select query, with the caveat that the return types must match the types expected by the insert. Currently, this is checked during query compilation rather than allowing the check to relegate to the database. This may cause problems between Hibernate Types which are equivalent as opposed to equal. For example, this might cause lead to issues with mismatches between an attribute mapped as a `org.hibernate.type.DateType` and an attribute defined as a `org.hibernate.type.TimestampType`, even though the database might not make a distinction or might be able to handle the conversion.

For the id attribute, the insert statement gives you two options. You can either explicitly specify the id property in the `attribute_list`, in which case its value is taken from the corresponding select expression, or omit it from the `attribute_list` in which case a generated value is used. This latter option is only available when using id generators that operate “in the database”; attempting to use this option with any “in memory” type generators will cause an exception during parsing.

For optimistic locking attributes, the insert statement again gives you two options. You can either specify the attribute in the `attribute_list` in which case its value is taken from the corresponding select expressions, or omit it from the `attribute_list` in which case the `seed value` defined by the corresponding `org.hibernate.type.VersionType` is used.

Example 11.2. Example INSERT query statements

    String hqlInsert = "insert into DelinquentAccount (id, name) select c.id, c.name from Customer c where ...";
    int createdEntities = s.createQuery( hqlInsert ).executeUpdate();

## 11.3. The `FROM` clause

The `FROM` clause is responsible defining the scope of object model types available to the rest of the query. It also is responsible for defining all the “identification variables” available to the rest of the query.

### 11.3.1. Identification variables

Identification variables are often referred to as aliases. References to object model classes in the FROM clause can be associated with an identification variable that can then be used to refer to that type thoughout the rest of the query.

In most cases declaring an identification variable is optional, though it is usually good practice to declare them.

An identification variable must follow the rules for Java identifier validity.

According to JPQL, identification variables must be treated as case insensitive. Good practice says you should use the same case throughout a query to refer to a given identification variable. In other words, JPQL says they can be case insensitive and so Hibernate must be able to treat them as such, but this does not make it good practice.

### 11.3.2. Root entity references

A root entity reference, or what JPA calls a `range variable declaration`, is specifically a reference to a mapped entity type from the application. It cannot name component/ embeddable types. And associations, including collections, are handled in a different manner discussed later.

The BNF for a root entity reference is:

    root_entity_reference ::= entity_name [AS] identification_variable

Example 11.3. Simple query example

    select c from com.acme.Cat c

We see that the query is defining a root entity reference to the `com.acme.Cat` object model type. Additionally, it declares an alias of `c` to that `com.acme.Cat` reference; this is the identification variable.

Usually the root entity reference just names the `entity name` rather than the entity class FQN. By default the entity name is the unqualified entity class name, here `Cat`

Example 11.4. Simple query using entity name for root entity reference

    select c from Cat c

Multiple root entity references can also be specified. Even naming the same entity!

Example 11.5. Simple query using multiple root entity references

    // build a product between customers and active mailing campaigns so we can spam!
    select distinct cust, camp
    from Customer cust, Campaign camp
    where camp.type = 'mail'
      and current_timestamp() between camp.activeRange.start and camp.activeRange.end

    // retrieve all customers with headquarters in the same state as Acme's headquarters
    select distinct c1
    from Customer c1, Customer c2
    where c1.address.state = c2.address.state
      and c2.name = 'Acme'

### 11.3.3. Explicit joins

The `FROM` clause can also contain explicit relationship joins using the `join` keyword. These joins can be either `inner` or `left outer` style joins.

Example 11.6. Explicit inner join examples

    select c
    from Customer c
        join c.chiefExecutive ceo
    where ceo.age < 25

    // same query but specifying join type as 'inner' explicitly
    select c
    from Customer c
        inner join c.chiefExecutive ceo
    where ceo.age < 25

Example 11.7. Explicit left (outer) join examples

    // get customers who have orders worth more than $5000
    // or who are in "preferred" status
    select distinct c
    from Customer c
        left join c.orders o
    where o.value > 5000.00
      or c.status = 'preferred'

    // functionally the same query but using the
    // 'left outer' phrase
    select distinct c
    from Customer c
        left outer join c.orders o
    where o.value > 5000.00
      or c.status = 'preferred'

An important use case for explicit joins is to define `FETCH JOINS` which override the laziness of the joined association. As an example, given an entity named `Customer` with a collection-valued association named `orders`

Example 11.8. Fetch join example

    select c
    from Customer c
        left join fetch c.orders o

As you can see from the example, a fetch join is specified by injecting the keyword `fetch` after the keyword `join`. In the example, we used a left outer join because we want to return customers who have no orders also. Inner joins can also be fetched. But inner joins still filter. In the example, using an inner join instead would have resulted in customers without any orders being filtered out of the result.

## Important

Fetch joins are not valid in sub-queries.

Care should be taken when fetch joining a collection-valued association which is in any way further restricted; the fetched collection will be restricted too! For this reason it is usually considered best practice to not assign an identification variable to fetched joins except for the purpose of specifying nested fetch joins.

Fetch joins should not be used in paged queries (aka, `setFirstResult`/ `setMaxResults`). Nor should they be used with the HQL `scroll` or `iterate` features.

HQL also defines a `WITH` clause to qualify the join conditions. Again, this is specific to HQL; JPQL does not define this feature.

Example 11.9. with-clause join example

    select distinct c
    from Customer c
        left join c.orders o
            with o.value > 5000.00

The important distinction is that in the generated SQL the conditions of the `with clause` are made part of the `on clause` in the generated SQL as opposed to the other queries in this section where the HQL/JPQL conditions are made part of the `where clause` in the generated SQL. The distinction in this specific example is probably not that significant. The `with clause` is sometimes necessary in more complicated queries.

Explicit joins may reference association or component/embedded attributes. For further information about collection-valued association references, see Section 11.3.5, “Collection member references”. In the case of component/embedded attributes, the join is simply logical and does not correlate to a physical (SQL) join.

### 11.3.4. Implicit joins (path expressions)

Another means of adding to the scope of object model types available to the query is through the use of implicit joins, or path expressions.

Example 11.10. Simple implicit join example

    select c
    from Customer c
    where c.chiefExecutive.age < 25

    // same as
    select c
    from Customer c
        inner join c.chiefExecutive ceo
    where ceo.age < 25

An implicit join always starts from an `identification variable`, followed by the navigation operator (.), followed by an attribute for the object model type referenced by the initial `identification variable`. In the example, the initial `identification variable` is `c` which refers to the `Customer` entity. The `c.chiefExecutive` reference then refers to the `chiefExecutive` attribute of the `Customer` entity. `chiefExecutive` is an association type so we further navigate to its `age` attribute.

## Important

If the attribute represents an entity association (non-collection) or a component/embedded, that reference can be further navigated. Basic values and collection-valued associations cannot be further navigated.

As shown in the example, implicit joins can appear outside the `FROM clause`. However, they affect the `FROM clause`. Implicit joins are always treated as inner joins. Multiple references to the same implicit join always refer to the same logical and physical (SQL) join.

Example 11.11. Reused implicit join

    select c
    from Customer c
    where c.chiefExecutive.age < 25
       and c.chiefExecutive.address.state = 'TX'

    // same as
    select c
    from Customer c
        inner join c.chiefExecutive ceo
    where ceo.age < 25
      and ceo.address.state = 'TX'

    // same as
    select c
    from Customer c
        inner join c.chiefExecutive ceo
        inner join ceo.address a
    where ceo.age < 25
      and a.state = 'TX'

Just as with explicit joins, implicit joins may reference association or component/embedded attributes. For further information about collection-valued association references, see Section 11.3.5, “Collection member references”. In the case of component/embedded attributes, the join is simply logical and does not correlate to a physical (SQL) join. Unlike explicit joins, however, implicit joins may also reference basic state fields as long as the path expression ends there.

### 11.3.5. Collection member references

References to collection-valued associations actually refer to the values of that collection.

Example 11.12. Collection references example

    select c
    from Customer c
        join c.orders o
        join o.lineItems l
        join l.product p
    where o.status = 'pending'
      and p.status = 'backorder'

    // alternate syntax
    select c
    from Customer c,
        in(c.orders) o,
        in(o.lineItems) l
        join l.product p
    where o.status = 'pending'
      and p.status = 'backorder'

In the example, the identification variable `o` actually refers to the object model type `Order` which is the type of the elements of the `Customer#orders` association.

The example also shows the alternate syntax for specifying collection association joins using the `IN` syntax. Both forms are equivalent. Which form an application chooses to use is simply a matter of taste.

#### 11.3.5.1. Special case - qualified path expressions

We said earlier that collection-valued associations actually refer to the values of that collection. Based on the type of collection, there are also available a set of explicit qualification expressions.

Example 11.13. Qualified collection references example

    // Product.images is a Map<String,String> : key = a name, value = file path

    // select all the image file paths (the map value) for Product#123
    select i
    from Product p
        join p.images i
    where p.id = 123

    // same as above
    select value(i)
    from Product p
        join p.images i
    where p.id = 123

    // select all the image names (the map key) for Product#123
    select key(i)
    from Product p
        join p.images i
    where p.id = 123

    // select all the image names and file paths (the 'Map.Entry') for Product#123
    select entry(i)
    from Product p
        join p.images i
    where p.id = 123

    // total the value of the initial line items for all orders for a customer
    select sum( li.amount )
    from Customer c
            join c.orders o
            join o.lineItems li
    where c.id = 123
      and index(li) = 1

VALUE

  Refers to the collection value. Same as not specifying a qualifier. Useful to explicitly show intent. Valid for any type of collection-valued reference.

INDEX

  According to HQL rules, this is valid for both Maps and Lists which specify a `javax.persistence.OrderColumn` annotation to refer to the Map key or the List position (aka the OrderColumn value). JPQL however, reserves this for use in the List case and adds `KEY` for the MAP case. Applications interested in JPA provider portability should be aware of this distinction.

KEY

  Valid only for Maps. Refers to the map's key. If the key is itself an entity, can be further navigated.

ENTRY

  Only valid only for Maps. Refers to the Map's logical `java.util.Map.Entry` tuple (the combination of its key and value). `ENTRY` is only valid as a terminal path and only valid in the select clause.

See Section 11.4.9, “Collection-related expressions” for additional details on collection related expressions.

### 11.3.6. Polymorphism

HQL and JPQL queries are inherently polymorphic.

    select p from Payment p

This query names the `Payment` entity explicitly. However, all subclasses of `Payment` are also available to the query. So if the `CreditCardPayment` entity and `WireTransferPayment` entity each extend from `Payment` all three types would be available to the query. And the query would return instances of all three.

## The logical extreme

The HQL query `from java.lang.Object` is totally valid! It returns every object of every type defined in your application.

This can be altered by using either the `org.hibernate.annotations.Polymorphism` annotation (global, and Hibernate-specific) or limiting them using in the query itself using an entity type expression.

## 11.4. Expressions

Essentially expressions are references that resolve to basic or tuple values.

### 11.4.1. Identification variable

See Section 11.3, “The `FROM` clause”.

### 11.4.2. Path expressions

Again, see Section 11.3, “The `FROM` clause”.

### 11.4.3. Literals

String literals are enclosed in single-quotes. To escape a single-quote within a string literal, use double single-quotes.

Example 11.14. String literal examples

    select c
    from Customer c
    where c.name = 'Acme'

    select c
    from Customer c
    where c.name = 'Acme''s Pretzel Logic'

Numeric literals are allowed in a few different forms.

Example 11.15. Numeric literal examples

    // simple integer literal
    select o
    from Order o
    where o.referenceNumber = 123

    // simple integer literal, typed as a long
    select o
    from Order o
    where o.referenceNumber = 123L

    // decimal notation
    select o
    from Order o
    where o.total > 5000.00

    // decimal notation, typed as a float
    select o
    from Order o
    where o.total > 5000.00F

    // scientific notation
    select o
    from Order o
    where o.total > 5e+3

    // scientific notation, typed as a float
    select o
    from Order o
    where o.total > 5e+3F

In the scientific notation form, the `E` is case insensitive.

Specific typing can be achieved through the use of the same suffix approach specified by Java. So, `L` denotes a long; `D` denotes a double; `F` denotes a float. The actual suffix is case insensitive.

The boolean literals are `TRUE` and `FALSE`, again case-insensitive.

Enums can even be referenced as literals. The fully-qualified enum class name must be used. HQL can also handle constants in the same manner, though JPQL does not define that as supported.

Entity names can also be used as literal. See Section 11.4.10, “Entity type”.

Date/time literals can be specified using the JDBC escape syntax: `{d 'yyyy-mm-dd'}` for dates, `{t 'hh:mm:ss'}` for times and `{ts 'yyyy-mm-dd hh:mm:ss[.millis]'}` (millis optional) for timestamps. These literals only work if you JDBC drivers supports them.

### 11.4.4. Parameters

HQL supports all 3 of the following forms. JPQL does not support the HQL-specific positional parameters notion. It is good practice to not mix forms in a given query.

#### 11.4.4.1. Named parameters

Named parameters are declared using a colon followed by an identifier - `:aNamedParameter`. The same named parameter can appear multiple times in a query.

Example 11.16. Named parameter examples

    String queryString =
            "select c " +
            "from Customer c " +
            "where c.name = :name " +
            "   or c.nickName = :name";

    // HQL
    List customers = session.createQuery( queryString )
            .setParameter( "name", theNameOfInterest )
            .list();

    // JPQL
    List<Customer> customers = entityManager.createQuery( queryString, Customer.class )
            .setParameter( "name", theNameOfInterest )
            .getResultList();

#### 11.4.4.2. Positional (JPQL) parameters

JPQL-style positional parameters are declared using a question mark followed by an ordinal - `?1`, `?2`. The ordinals start with 1. Just like with named parameters, positional parameters can also appear multiple times in a query.

Example 11.17. Positional (JPQL) parameter examples

    String queryString =
            "select c " +
            "from Customer c " +
            "where c.name = ?1 " +
            "   or c.nickName = ?1";

    // HQL - as you can see, handled just like named parameters
    //      in terms of API
    List customers = session.createQuery( queryString )
            .setParameter( "1", theNameOfInterest )
            .list();

    // JPQL
    List<Customer> customers = entityManager.createQuery( queryString, Customer.class )
            .setParameter( 1, theNameOfInterest )
            .getResultList();

#### 11.4.4.3. Positional (HQL) parameters

HQL-style positional parameters follow JDBC positional parameter syntax. They are declared using `?` without a following ordinal. There is no way to relate two such positional parameters as being "the same" aside from binding the same value to each.

This form should be considered deprecated and may be removed in the near future.

### 11.4.5. Arithmetic

Arithmetic operations also represent valid expressions.

Example 11.18. Numeric arithmetic examples

    select year( current_date() ) - year( c.dateOfBirth )
    from Customer c

    select c
    from Customer c
    where year( current_date() ) - year( c.dateOfBirth ) < 30

    select o.customer, o.total + ( o.total * :salesTax )
    from Order o

The following rules apply to the result of arithmetic operations:

- If either of the operands is Double/double, the result is a Double;

- else, if either of the operands is Float/float, the result is a Float;

- else, if either operand is BigDecimal, the result is BigDecimal;

- else, if either operand is BigInteger, the result is BigInteger (except for division, in which case the result type is not further defined);

- else, if either operand is Long/long, the result is Long (except for division, in which case the result type is not further defined);

- else, (the assumption being that both operands are of integral type) the result is Integer (except for division, in which case the result type is not further defined);

Date arithmetic is also supported, albeit in a more limited fashion. This is due partially to differences in database support and partially to the lack of support for `INTERVAL` definition in the query language itself.

### 11.4.6. Concatenation (operation)

HQL defines a concatenation operator in addition to supporting the concatenation (`CONCAT`) function. This is not defined by JPQL, so portable applications should avoid it use. The concatenation operator is taken from the SQL concatenation operator - `||`.

Example 11.19. Concatenation operation example

    select 'Mr. ' || c.name.first || ' ' || c.name.last
    from Customer c
    where c.gender = Gender.MALE

See Section 11.4.8, “Scalar functions” for details on the `concat()` function

### 11.4.7. Aggregate functions

Aggregate functions are also valid expressions in HQL and JPQL. The semantic is the same as their SQL counterpart. The supported aggregate functions are:

- `COUNT` (including distinct/all qualifiers) - The result type is always Long.

- `AVG` - The result type is always Double.

- `MIN` - The result type is the same as the argument type.

- `MAX` - The result type is the same as the argument type.

- `SUM` - The result type of the `avg()` function depends on the type of the values being averaged. For integral values (other than BigInteger), the result type is Long. For floating point values (other than BigDecimal) the result type is Double. For BigInteger values, the result type is BigInteger. For BigDecimal values, the result type is BigDecimal.

Example 11.20. Aggregate function examples

    select count(*), sum( o.total ), avg( o.total ), min( o.total ), max( o.total )
    from Order o

    select count( distinct c.name )
    from Customer c

    select c.id, c.name, sum( o.total )
    from Customer c
        left join c.orders o
    group by c.id, c.name

Aggregations often appear with grouping. For information on grouping see Section 11.8, “Grouping”

### 11.4.8. Scalar functions

Both HQL and JPQL define some standard functions that are available regardless of the underlying database in use. HQL can also understand additional functions defined by the Dialect as well as the application.

#### 11.4.8.1. Standardized functions - JPQL

Here are the list of functions defined as supported by JPQL. Applications interested in remaining portable between JPA providers should stick to these functions.

CONCAT

  String concatenation function. Variable argument length of 2 or more string values to be concatenated together.

SUBSTRING

  Extracts a portion of a string value.

      substring( string_expression, numeric_expression [, numeric_expression] )

  The second argument denotes the starting position. The third (optional) argument denotes the length.

UPPER

  Upper cases the specified string

LOWER

  Lower cases the specified string

TRIM

  Follows the semantics of the SQL trim function.

LENGTH

  Returns the length of a string.

LOCATE

  Locates a string within another string.

      locate( string_expression, string_expression[, numeric_expression] )

  The third argument (optional) is used to denote a position from which to start looking.

ABS

  Calculates the mathematical absolute value of a numeric value.

MOD

  Calculates the remainder of dividing the first argument by the second.

SQRT

  Calculates the mathematical square root of a numeric value.

CURRENT_DATE

  Returns the database current date.

CURRENT_TIME

  Returns the database current time.

CURRENT_TIMESTAMP

  Returns the database current timestamp.

#### 11.4.8.2. Standardized functions - HQL

Beyond the JPQL standardized functions, HQL makes some additional functions available regardless of the underlying database in use.

BIT_LENGTH

  Returns the length of binary data.

CAST

  Performs a SQL cast. The cast target should name the Hibernate mapping type to use. See the chapter on data types for more information.

EXTRACT

  Performs a SQL extraction on datetime values. An extraction extracts parts of the datetime (the year, for example). See the abbreviated forms below.

SECOND

  Abbreviated extract form for extracting the second.

MINUTE

  Abbreviated extract form for extracting the minute.

HOUR

  Abbreviated extract form for extracting the hour.

DAY

  Abbreviated extract form for extracting the day.

MONTH

  Abbreviated extract form for extracting the month.

YEAR

  Abbreviated extract form for extracting the year.

STR

  Abbreviated form for casting a value as character data.

#### 11.4.8.3. Non-standardized functions

Hibernate Dialects can register additional functions known to be available for that particular database product. These functions are also available in HQL (and JPQL, though only when using Hibernate as the JPA provider obviously). However, they would only be available when using that database/Dialect. Applications that aim for database portability should avoid using functions in this category.

Application developers can also supply their own set of functions. This would usually represent either custom SQL functions or aliases for snippets of SQL. Such function declarations are made by using the `addSqlFunction` method of `org.hibernate.cfg.Configuration`

### 11.4.9. Collection-related expressions

There are a few specialized expressions for working with collection-valued associations. Generally these are just abbreviated forms or other expressions for the sake of conciseness.

SIZE

  Calculate the size of a collection. Equates to a subquery!

MAXELEMENT

  Available for use on collections of basic type. Refers to the maximum value as determined by applying the `max` SQL aggregation.

MAXINDEX

  Available for use on indexed collections. Refers to the maximum index (key/position) as determined by applying the `max` SQL aggregation.

MINELEMENT

  Available for use on collections of basic type. Refers to the minimum value as determined by applying the `min` SQL aggregation.

MININDEX

  Available for use on indexed collections. Refers to the minimum index (key/position) as determined by applying the `min` SQL aggregation.

ELEMENTS

  Used to refer to the elements of a collection as a whole. Only allowed in the where clause. Often used in conjunction with `ALL`, `ANY` or `SOME` restrictions.

INDICES

  Similar to `elements` except that `indices` refers to the collections indices (keys/positions) as a whole.

Example 11.21. Collection-related expressions examples

    select cal
    from Calendar cal
    where maxelement(cal.holidays) > current_date()

    select o
    from Order o
    where maxindex(o.items) > 100

    select o
    from Order o
    where minelement(o.items) > 10000

    select m
    from Cat as m, Cat as kit
    where kit in elements(m.kittens)

    // the above query can be re-written in jpql standard way:
    select m
    from Cat as m, Cat as kit
    where kit member of m.kittens

    select p
    from NameList l, Person p
    where p.name = some elements(l.names)

    select cat
    from Cat cat
    where exists elements(cat.kittens)

    select p
    from Player p
    where 3 > all elements(p.scores)

    select show
    from Show show
    where 'fizard' in indices(show.acts)

Elements of indexed collections (arrays, lists, and maps) can be referred to by index operator.

Example 11.22. Index operator examples

    select o
    from Order o
    where o.items[0].id = 1234

    select p
    from Person p, Calendar c
    where c.holidays['national day'] = p.birthDay
      and p.nationality.calendar = c

    select i
    from Item i, Order o
    where o.items[ o.deliveredItemIndices[0] ] = i
      and o.id = 11

    select i
    from Item i, Order o
    where o.items[ maxindex(o.items) ] = i
      and o.id = 11

    select i
    from Item i, Order o
    where o.items[ size(o.items) - 1 ] = i

See also Section 11.3.5.1, “Special case - qualified path expressions” as there is a good deal of overlap.

### 11.4.10. Entity type

We can also refer to the type of an entity as an expression. This is mainly useful when dealing with entity inheritance hierarchies. The type can expressed using a `TYPE` function used to refer to the type of an identification variable representing an entity. The name of the entity also serves as a way to refer to an entity type. Additionally the entity type can be parametrized, in which case the entity's Java Class reference would be bound as the parameter value.

Example 11.23. Entity type expression examples

    select p
    from Payment p
    where type(p) = CreditCardPayment

    select p
    from Payment p
    where type(p) = :aType

HQL also has a legacy form of referring to an entity type, though that legacy form is considered deprecated in favor of `TYPE`. The legacy form would have used `p.class` in the examples rather than `type(p)`. It is mentioned only for completeness.

### 11.4.11. CASE expressions

Both the simple and searched forms are supported, as well as the 2 SQL defined abbreviated forms (`NULLIF` and `COALESCE`)

#### 11.4.11.1. Simple CASE expressions

The simple form has the following syntax:

    CASE {operand} WHEN {test_value} THEN {match_result} ELSE {miss_result} END

Example 11.24. Simple case expression example

    select case c.nickName when null then '<no nick name>' else c.nickName end
    from Customer c

    // This NULL checking is such a common case that most dbs
    // define an abbreviated CASE form.  For example:
    select nvl( c.nickName, '<no nick name>' )
    from Customer c

    // or:
    select isnull( c.nickName, '<no nick name>' )
    from Customer c

    // the standard coalesce abbreviated form can be used
    // to achieve the same result:
    select coalesce( c.nickName, '<no nick name>' )
    from Customer c

#### 11.4.11.2. Searched CASE expressions

The searched form has the following syntax:

    CASE [ WHEN {test_conditional} THEN {match_result} ]* ELSE {miss_result} END

Example 11.25. Searched case expression example

    select case when c.name.first is not null then c.name.first
                when c.nickName is not null then c.nickName
                else '<no first name>' end
    from Customer c

    // Again, the abbreviated form coalesce can handle this a
    // little more succinctly
    select coalesce( c.name.first, c.nickName, '<no first name>' )
    from Customer c

#### 11.4.11.3. NULLIF expressions

NULLIF is an abbreviated CASE expression that returns NULL if its operands are considered equal.

Example 11.26. NULLIF example

    // return customers who have changed their last name
    select nullif( c.previousName.last, c.name.last )
    from Customer c

    // equivalent CASE expression
    select case when c.previousName.last = c.name.last then null
                else c.previousName.last end
    from Customer c

#### 11.4.11.4. COALESCE expressions

COALESCE is an abbreviated CASE expression that returns the first non-null operand. We have seen a number of COALESCE examples above.

## 11.5. The `SELECT` clause

The `SELECT` clause identifies which objects and values to return as the query results. The expressions discussed in Section 11.4, “Expressions” are all valid select expressions, except where otherwise noted. See the section Section 11.10, “Query API” for information on handling the results depending on the types of values specified in the `SELECT` clause.

There is a particular expression type that is only valid in the select clause. Hibernate calls this “dynamic instantiation”. JPQL supports some of that feature and calls it a “constructor expression”

Example 11.27. Dynamic instantiation example - constructor

    select new Family( mother, mate, offspr )
    from DomesticCat as mother
        join mother.mate as mate
        left join mother.kittens as offspr

So rather than dealing with the Object[] (again, see Section 11.10, “Query API”) here we are wrapping the values in a type-safe java object that will be returned as the results of the query. The class reference must be fully qualified and it must have a matching constructor.

The class here need not be mapped. If it does represent an entity, the resulting instances are returned in the NEW state (not managed!).

That is the part JPQL supports as well. HQL supports additional “dynamic instantiation” features. First, the query can specify to return a List rather than an Object[] for scalar results:

Example 11.28. Dynamic instantiation example - list

    select new list(mother, offspr, mate.name)
    from DomesticCat as mother
        inner join mother.mate as mate
        left outer join mother.kittens as offspr

The results from this query will be a List<List> as opposed to a List<Object[]>

HQL also supports wrapping the scalar results in a Map.

Example 11.29. Dynamic instantiation example - map

    select new map( mother as mother, offspr as offspr, mate as mate )
    from DomesticCat as mother
        inner join mother.mate as mate
        left outer join mother.kittens as offspr

    select new map( max(c.bodyWeight) as max, min(c.bodyWeight) as min, count(*) as n )
    from Cat c

The results from this query will be a List<Map<String,Object>> as opposed to a List<Object[]>. The keys of the map are defined by the aliases given to the select expressions.

## 11.6. Predicates

Predicates form the basis of the where clause, the having clause and searched case expressions. They are expressions which resolve to a truth value, generally `TRUE` or `FALSE`, although boolean comparisons involving NULLs generally resolve to `UNKNOWN`.

### 11.6.1. Relational comparisons

Comparisons involve one of the comparison operators - =, >, >=, <, <=, <>]>. HQL also defines <![CDATA[!= as a comparison operator synonymous with <>. The operands should be of the same type.

Example 11.30. Relational comparison examples

    // numeric comparison
    select c
    from Customer c
    where c.chiefExecutive.age < 30

    // string comparison
    select c
    from Customer c
    where c.name = 'Acme'

    // datetime comparison
    select c
    from Customer c
    where c.inceptionDate < {d '2000-01-01'}

    // enum comparison
    select c
    from Customer c
    where c.chiefExecutive.gender = com.acme.Gender.MALE

    // boolean comparison
    select c
    from Customer c
    where c.sendEmail = true

    // entity type comparison
    select p
    from Payment p
    where type(p) = WireTransferPayment

    // entity value comparison
    select c
    from Customer c
    where c.chiefExecutive = c.chiefTechnologist

Comparisons can also involve subquery qualifiers - `ALL`, `ANY`, `SOME`. SOME and ANY are synonymous.

The ALL qualifier resolves to true if the comparison is true for all of the values in the result of the subquery. It resolves to false if the subquery result is empty.

Example 11.31. ALL subquery comparison qualifier example

    // select all players that scored at least 3 points
    // in every game.
    select p
    from Player p
    where 3 > all (
        select spg.points
        from StatsPerGame spg
        where spg.player = p
    )

The ANY/SOME qualifier resolves to true if the comparison is true for some of (at least one of) the values in the result of the subquery. It resolves to false if the subquery result is empty.

### 11.6.2. Nullness predicate

Check a value for nullness. Can be applied to basic attribute references, entity references and parameters. HQL additionally allows it to be applied to component/embeddable types.

Example 11.32. Nullness checking examples

    // select everyone with an associated address
    select p
    from Person p
    where p.address is not null

    // select everyone without an associated address
    select p
    from Person p
    where p.address is null

### 11.6.3. Like predicate

Performs a like comparison on string values. The syntax is:

    like_expression ::=
            string_expression
            [NOT] LIKE pattern_value
            [ESCAPE escape_character]

The semantics follow that of the SQL like expression. The `pattern_value` is the pattern to attempt to match in the `string_expression`. Just like SQL, `pattern_value` can use “_” and “%” as wildcards. The meanings are the same. “_” matches any single character. “%” matches any number of characters.

The optional `escape_character` is used to specify an escape character used to escape the special meaning of “_” and “%” in the `pattern_value`. THis is useful when needing to search on patterns including either “_” or “%”

Example 11.33. Like predicate examples

    select p
    from Person p
    where p.name like '%Schmidt'

    select p
    from Person p
    where p.name not like 'Jingleheimmer%'

    // find any with name starting with "sp_"
    select sp
    from StoredProcedureMetadata sp
    where sp.name like 'sp|_%' escape '|'

### 11.6.4. Between predicate

Analogous to the SQL between expression. Perform a evaluation that a value is within the range of 2 other values. All the operands should have comparable types.

Example 11.34. Between predicate examples

    select p
    from Customer c
        join c.paymentHistory p
    where c.id = 123
      and index(p) between 0 and 9

    select c
    from Customer c
    where c.president.dateOfBirth
            between {d '1945-01-01'}
                and {d '1965-01-01'}

    select o
    from Order o
    where o.total between 500 and 5000

    select p
    from Person p
    where p.name between 'A' and 'E'

### 11.6.5. In predicate

`IN` predicates performs a check that a particular value is in a list of values. Its syntax is:

    in_expression ::= single_valued_expression
                [NOT] IN single_valued_list

    single_valued_list ::= constructor_expression |
                (subquery) |
                collection_valued_input_parameter

    constructor_expression ::= (expression[, expression]*)

The types of the `single_valued_expression` and the individual values in the `single_valued_list` must be consistent. JPQL limits the valid types here to string, numeric, date, time, timestamp, and enum types. In JPQL, `single_valued_expression` can only refer to:

- “state fields”, which is its term for simple attributes. Specifically this excludes association and component/embedded attributes.

- entity type expressions. See Section 11.4.10, “Entity type”

In HQL, `single_valued_expression` can refer to a far more broad set of expression types. Single-valued association are allowed. So are component/embedded attributes, although that feature depends on the level of support for tuple or “row value constructor syntax” in the underlying database. Additionally, HQL does not limit the value type in any way, though application developers should be aware that different types may incur limited support based on the underlying database vendor. This is largely the reason for the JPQL limitations.

The list of values can come from a number of different sources. In the `constructor_expression` and `collection_valued_input_parameter`, the list of values must not be empty; it must contain at least one value.

Example 11.35. In predicate examples

    select p
    from Payment p
    where type(p) in (CreditCardPayment, WireTransferPayment)

    select c
    from Customer c
    where c.hqAddress.state in ('TX', 'OK', 'LA', 'NM')

    select c
    from Customer c
    where c.hqAddress.state in ?

    select c
    from Customer c
    where c.hqAddress.state in (
        select dm.state
        from DeliveryMetadata dm
        where dm.salesTax is not null
    )

    // Not JPQL compliant!
    select c
    from Customer c
    where c.name in (
        ('John','Doe'),
        ('Jane','Doe')
    )

    // Not JPQL compliant!
    select c
    from Customer c
    where c.chiefExecutive in (
        select p
        from Person p
        where ...
    )

### 11.6.6. Exists predicate

Exists expressions test the existence of results from a subquery. The affirmative form returns true if the subquery result contains values. The negated form returns true if the subquery result is empty.

### 11.6.7. Empty collection predicate

The `IS [NOT] EMPTY` expression applies to collection-valued path expressions. It checks whether the particular collection has any associated values.

Example 11.36. Empty collection expression examples

    select o
    from Order o
    where o.lineItems is empty

    select c
    from Customer c
    where c.pastDueBills is not empty

### 11.6.8. Member-of collection predicate

The `[NOT] MEMBER [OF]` expression applies to collection-valued path expressions. It checks whether a value is a member of the specified collection.

Example 11.37. Member-of collection expression examples

    select p
    from Person p
    where 'John' member of p.nickNames

    select p
    from Person p
    where p.name.first = 'Joseph'
      and 'Joey' not member of p.nickNames

### 11.6.9. NOT predicate operator

The `NOT` operator is used to negate the predicate that follows it. If that following predicate is true, the NOT resolves to false. If the predicate is true, NOT resolves to false. If the predicate is unknown, the NOT resolves to unknown as well.

### 11.6.10. AND predicate operator

The `AND` operator is used to combine 2 predicate expressions. The result of the AND expression is true if and only if both predicates resolve to true. If either predicate resolves to unknown, the AND expression resolves to unknown as well. Otherwise, the result is false.

### 11.6.11. OR predicate operator

The `OR` operator is used to combine 2 predicate expressions. The result of the OR expression is true if either predicate resolves to true. If both predicates resolve to unknown, the OR expression resolves to unknown. Otherwise, the result is false.

## 11.7. The `WHERE` clause

The `WHERE` clause of a query is made up of predicates which assert whether values in each potential row match the predicated checks. Thus, the where clause restricts the results returned from a select query and limits the scope of update and delete queries.

## 11.8. Grouping

The `GROUP BY` clause allows building aggregated results for various value groups. As an example, consider the following queries:

Example 11.38. Group-by illustration

    // retrieve the total for all orders
    select sum( o.total )
    from Order o

    // retrieve the total of all orders
    // *grouped by* customer
    select c.id, sum( o.total )
    from Order o
        inner join o.customer c
    group by c.id

The first query retrieves the complete total of all orders. The second retrieves the total for each customer; grouped by each customer.

In a grouped query, the where clause applies to the non aggregated values (essentially it determines whether rows will make it into the aggregation). The `HAVING` clause also restricts results, but it operates on the aggregated values. In the Example 11.38, “Group-by illustration” example, we retrieved order totals for all customers. If that ended up being too much data to deal with, we might want to restrict the results to focus only on customers with a summed order total of more than $10,000.00:

Example 11.39. Having illustration

    select c.id, sum( o.total )
    from Order o
        inner join o.customer c
    group by c.id
    having sum( o.total ) > 10000.00

The HAVING clause follows the same rules as the WHERE clause and is also made up of predicates. HAVING is applied after the groupings and aggregations have been done; WHERE is applied before.

## 11.9. Ordering

The results of the query can also be ordered. The `ORDER BY` clause is used to specify the selected values to be used to order the result. The types of expressions considered valid as part of the order-by clause include:

- state fields

- component/embeddable attributes

- scalar expressions such as arithmetic operations, functions, etc.

- identification variable declared in the select clause for any of the previous expression types

Additionally, JPQL says that all values referenced in the order-by clause must be named in the select clause. HQL does not mandate that restriction, but applications desiring database portability should be aware that not all databases support referencing values in the order-by clause that are not referenced in the select clause.

Individual expressions in the order-by can be qualified with either `ASC` (ascending) or `DESC` (descending) to indicated the desired ordering direction.

Example 11.40. Order-by examples

    // legal because p.name is implicitly part of p
    select p
    from Person p
    order by p.name

    select c.id, sum( o.total ) as t
    from Order o
        inner join o.customer c
    group by c.id
    order by t

## 11.10. Query API

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