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DELETE entityName indentifierVariable WHERE where_clause
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For example, if we want to remove all instances of Seller with Silver status we d use this:
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DELETE Seller s WHERE s.status = 'Silver'
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Using the query API and JPQL to retrieve entities
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10.3.2 Using the FROM clause
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The FROM clause of JPQL is by far the most important clause. It defines the domain for the query that is, the names for the entities that will be used in the query. In the previous example we specified the FROM clause as follows:
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FROM Category c
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Category is the domain that we want to query, and here we have specified c as an identifier of type Category.
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Identifying the query domain: naming an entity You specify the entity name defined for the entity using the @Entity annotation as the domain type. As you learned in chapter 7, you could define the name for an entity using the name element of the @Entity annotation. If you don t specify the name element, it defaults to the name of the entity class. The name of entity must be unique within a persistence unit. In other words, you cannot have two entities with the same name or the persistence provider will generate a deployment error. This makes sense because the persistence provider would not be able to identify which entity domain to use if duplicate names for entities are allowed.
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EJB 2 s EJBQL compared to EJB 3 s JPQL
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If you have used EJBQL with EJB 2, you ll see a significant difference with the new version. Some of the major enhancements in EJB 3 with respect to JPQL are as follows:
Use of named parameters Simplification of syntax Support for JOIN operations Support for subqueries Bulk updates and deletes Support for GROUP BY and HAVING
The good news is that these JPQL enhancements may also be available to EJB 2.1 entity beans, because most application servers probably share the same JPQL parser for both EJB 2.1 entity beans and EJB 3 entities. One of the greatest shortcomings of EJB 2 CMP entity beans is the inability to use dynamic queries and native SQL. Both of these limitations have been addressed by JPQL.
Introducing JPQL
In the previous example, we are assuming the Category entity class that we discussed in earlier chapters does not define a name. If we assume that the Category class defines an entity name using the name element as follows:
@Entity(name = "CategoryEntity") public class Category
then we must change the FROM clause of the query as follows:
FROM CategoryEntity c
This change is required in order for JPQL to map the correct entity type as defined by the annotation. Identifier variables In our JPQL example, we defined an identifier variable named c, and we used that variable in other clauses, such as SELECT and WHERE. A simple identifier variable is defined using the following general syntax:
FROM entityName [AS] identificationVariable
The square brackets ([]) indicate that the AS operator is optional. The identifier variable (which is not case sensitive) must be a valid Java identifier, and it must not be a JPQL reserved identifier. Table 10.7 lists all of the JPQL reserved identifiers for your convenience. Keep in mind that the identifier cannot be another entity name packaged in the same persistence unit.
Table 10.7 JPQL keywords reserved by the specification. You are not allowed to give any of your variables these names. Types Statements and clauses Reserved Words SELECT, UPDATE, DELETE, FROM, WHERE, GROUP, HAVING, ORDER, BY, ASC, DESC JOIN, OUTER, INNER, LEFT, FETCH DISTINCT, OBJECT, NULL, TRUE, FALSE, NOT, AND, OR, BETWEEN, LIKE, IN, AS, UNKNOWN, EMPTY, MEMBER, OF, IS, NEW, EXISTS, ALL, ANY, SOME AVG, MAX, MIN, SUM, COUNT, MOD, UPPER, LOWER, TRIM, POSITION, CHARACTER_LENGTH, CHAR_LENGTH, BIT_LENGTH, CURRENT_TIME, CURRENT_DATE, CURRENT_TIMESTAMP
Joins Conditions and operators
Functions
Using the query API and JPQL to retrieve entities
Thus, we cannot define the FROM clause like this:
FROM Category User
or like this:
FROM Category Max
because we already have an entity named User in the ActionBazaar application, and MAX is a reserved identifier. You can define multiple identifiers in the FROM clause, and you ll see how to use them when we discuss joining multiple entities by association or field name later in this chapter. What is a path expression In our JPQL example we used expressions such as c.categoryName and c.categoryId. Such expressions are known as path expressions. A path expression is an identifier variable followed by the navigation operator (.), and a persistence or association field. We normally use a path expression to narrow the domain for a query by using it in a WHERE clause, or order the retrieved result by using an ORDER BY clause. An association field can contain either a single-value object or a collection. The association fields that represent one-to-many and many-to-many associations are collections of types, and such a path expression is a collection-value path expression. For example, if we have a many-to-many relationship between Category and Item, we can utilize a query to find all Category entities that have associated Items as follows:
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