Chapter 4. Hibernate Query Language

4.1. About Hibernate Query Language

Introduction to JPQL

The Java Persistence Query Language (JPQL) is a platform-independent object-oriented query language defined as part of the Java Persistence API (JPA) specification. JPQL is used to make queries against entities stored in a relational database. It is heavily inspired by SQL, and its queries resemble SQL queries in syntax, but operate against JPA entity objects rather than directly with database tables.

Introduction to HQL

The Hibernate Query Language (HQL) is a powerful query language, similar in appearance to SQL. Compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

HQL is a superset of JPQL. An HQL query is not always a valid JPQL query, but a JPQL query is always a valid HQL query.

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

4.2. About HQL Statements

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

The following table shows the syntax in Backus-Naur Form (BNF) notation for the various HQL statements.

Table 4.1. HQL Statements

StatementDescription

SELECT

The BNF for SELECT statements in HQL is:

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

UPDATE

The BNF for UPDATE statement in HQL is the same as it is in 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

DELETE

The BNF for DELETE statements in HQL is the same as it is in JPQL.

delete_statement ::= delete_clause [where_clause]

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

INSERT

The BNF for INSERT statement in HQL is:

insert_statement ::= insert_clause select_statement

insert_clause ::= INSERT INTO entity_name (attribute_list)

attribute_list ::= state_field[, state_field ]*

There is no JPQL equivalent to this.

Warning

Hibernate allows the use of Data Manipulation Language (DML) to bulk insert, update and delete data directly in the mapped database through the Hibernate Query Language (HQL).

Using DML may violate the object/relational mapping and may affect object state. Object state stays in memory and by using DML, the state of an in-memory object is not affected, depending on the operation that is performed on the underlying database. In-memory data must be used with care if DML is used.

About the UPDATE and DELETE Statements

The pseudo-syntax for UPDATE and DELETE statements is:

( UPDATE | DELETE ) FROM? EntityName (WHERE where_conditions)?.

Note

The FROM keyword and the WHERE Clause are optional. The FROM clause is responsible for 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. The WHERE clause allows you to refine the list of instances returned.

The result of execution of a UPDATE or DELETE statement is the number of rows that are actually affected (updated or deleted).

Example: Bulk Update Statement

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlUpdate = "update Company set name = :newName where name = :oldName";
int updatedEntities = s.createQuery( hqlUpdate )
        .setString( "newName", newName )
        .setString( "oldName", oldName )
        .executeUpdate();
tx.commit();
session.close();

Example: Bulk Delete Statement

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlDelete = "delete Company where name = :oldName";
int deletedEntities = s.createQuery( hqlDelete )
        .setString( "oldName", oldName )
        .executeUpdate();
tx.commit();
session.close();

The int value returned by the Query.executeUpdate() method indicates the number of entities within the database that were affected by the operation.

Internally, the database might use multiple SQL statements to execute the operation in response to a DML Update or Delete request. This might be because of relationships that exist between tables and the join tables that need to be updated or deleted.

For example, issuing a delete statement, as in the example above, may actually result in deletes being executed against not just the Company table for companies that are named with oldName, but also against joined tables. Therefore a Company table in a bidirectional, many-to-many relationship with an Employee table would also lose rows from the corresponding join table, Company_Employee, as a result of the successful execution of the previous example.

The deletedEntries value above will contain a count of all the rows affected due to this operation, including the rows in the join tables.

Important

Care should be taken 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 context or before fetching or accessing entities whose state might be affected by such operations.

About the INSERT Statement

HQL adds the ability to define INSERT statements. There is no JPQL equivalent to this. The Backus-Naur Form (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.

Warning

The 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 can cause problems with Hibernate Types that are equivalent as opposed to equal. For example, this might cause mismatch issues between an attribute mapped as an 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: 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();

Example: Bulk Insert Statement

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlInsert = "insert into Account (id, name) select c.id, c.name from Customer c where ...";
int createdEntities = s.createQuery( hqlInsert )
        .executeUpdate();
tx.commit();
session.close();

If you do not supply the value for the id attribute using the SELECT statement, an identifier is generated for you, as long as the underlying database supports auto-generated keys. The return value of this bulk insert operation is the number of entries actually created in the database.

4.3. About HQL 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

HQL does not mandate that all values referenced in the order-by clause must be named in the select clause, but it is required by JPQL. 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 indicate the desired ordering direction.

Example: Order By

// 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

4.4. About Collection Member References

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

Example: Collection References

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.

4.5. About Qualified Path Expressions

It was previously stated 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.

Table 4.2. Qualified Path Expressions

ExpressionDescription

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.

Example: Qualified Collection References

// 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

4.6. About HQL Functions

HQL defines some standard functions that are available regardless of the underlying database in use. HQL can also understand additional functions defined by the dialect and the application.

4.6.1. About HQL Standardized Functions

The following functions are available in HQL regardless of the underlying database in use.

Table 4.3. HQL Standardized Functions

FunctionDescription

BIT_LENGTH

Returns the length of binary data.

CAST

Performs an SQL cast. The cast target should name the Hibernate mapping type to use.

EXTRACT

Performs an SQL extraction on datetime values. An extraction returns a part of the date/time value, for example, the year. 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.

4.6.2. About HQL Non-Standardized Functions

Hibernate dialects can register additional functions known to be available for that particular database product. They would only be available when using that database or 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.

4.6.3. About the 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 using it. The concatenation operator is taken from the SQL concatenation operator (||).

Example: Concatenation Operation Example

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

4.7. About Dynamic Instantiation

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

Example: 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[] 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 does not need to be mapped. If it does represent an entity, the resulting instances are returned in the NEW state (not managed!).

This 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: 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: 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 cxt

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.

4.8. About HQL 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 NULL values generally resolve to UNKNOWN.

HQL Predicates

  • Null Predicate

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

    Example: NULL Check

    // 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

  • 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 _ (underscore) and % (percent) as wildcards. The meanings are the same. The _ matches any single character. The % 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: LIKE Predicate

    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 '|'

  • Between Predicate

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

    Example: BETWEEN Predicate

    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'

  • IN Predicate

    The IN predicate 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.

      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: IN Predicate

      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 ...
      )

4.9. About Relational Comparisons

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

Example: 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: 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 at least one of the values in the result of the subquery. It resolves to false if the subquery result is empty.

4.10. Bytecode Enhancement

4.10.1. Lazy Attribute Loading

Lazy attribute loading is a bytecode enhancement which allows you to tell Hibernate that only certain parts of an entity should be loaded upon fetching from the database, and when the other remaining parts should be loaded as well. This is different from proxy-based idea of lazy loading which is entity-centric where the entity’s state is loaded at once as needed. With bytecode enhancement, individual attributes or groups of attributes are loaded as needed.

Lazy attributes can be designated to be loaded together and this is called a lazy group. By default, all singular attributes are part of a single group. When one lazy singular attribute is accessed, all lazy singular attributes are loaded. Contrary to lazy singular group, lazy plural attributes are each a discrete lazy group. This behavior is explicitly controllable through the @org.hibernate.annotations.LazyGroup annotation.

@Entity
public class Customer {

    @Id
    private Integer id;

    private String name;

    @Basic( fetch = FetchType.LAZY )
    private UUID accountsPayableXrefId;

    @Lob
    @Basic( fetch = FetchType.LAZY )
    @LazyGroup( "lobs" )
    private Blob image;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public UUID getAccountsPayableXrefId() {
        return accountsPayableXrefId;
    }

    public void setAccountsPayableXrefId(UUID accountsPayableXrefId) {
        this.accountsPayableXrefId = accountsPayableXrefId;
    }

    public Blob getImage() {
        return image;
    }

    public void setImage(Blob image) {
        this.image = image;
    }
}

In the example above, there are two lazy attributes: accountsPayableXrefId and image. Each of these attributes is part of a different fetch group. The accountsPayableXrefId attribute is a part of the default fetch group, which means that accessing accountsPayableXrefId will not force the loading of the image attribute, and vice versa.