Joins, reporting queries, and (Post office web site) subselects HQL and JPA
Joins, reporting queries, and subselects HQL and JPA QL join options In Hibernate queries, you don t usually specify a join condition explicitly. Rather, you specify the name of a mapped Java class association. This is basically the same feature we d prefer to have in SQL, a join condition expressed with a foreign key constraint name. Because you ve mapped most, if not all, foreign key relationships of your database schema in Hibernate, you can use the names of these mapped associations in the query language. This is really syntactical sugar, but it s convenient. For example, the Item class has an association named bids with the Bid class. If you name this association in a query, Hibernate has enough information in the mapping document to then deduce the table join expression. This helps make queries less verbose and more readable. In fact, HQL and JPA QL provide four ways of expressing (inner and outer) joins: An implicit association join An ordinary join in the FROM clause A fetch join in the FROM clause A theta-style join in the WHERE clause Later we show you how to write a join between two classes that don t have an association defined (a theta-style join) and how to write ordinary and fetch joins in the FROM clause of a query. Implicit association joins are common abbreviations. (Note that we decided to make the following examples easier to read and understand by often omitting the SELECT clause valid in HQL, invalid in JPA QL.) Implicit association joins So far, you ve used simple qualified property names like bid.amount and item.description in queries. HQL and JPA QL support multipart property path expressions with a dot notation for two different purposes: Querying components Expressing implicit association joins The first use is straightforward: from User u where u.homeAddress.city = ‘Bangkok’ You reference parts of the mapped component Address with a dot notation. No tables are joined in this query; the properties of the homeAddress component are
We recommend high quality webhost to host and run your jsp application: christian web host services.