Chapter 9. Queries and EJB QL
Querying is a fundamental feature of all relational databases. It allows you to pull complex reports, calculations, and information about intricately related objects from persistence storage. Queries in Java Persistence are done using both the EJB QL query language and native Structured Query Language (SQL).
EJB QL is a declarative query language similar to the SQL used in relational databases, but it is tailored to work with Java objects rather than a relational schema. To execute queries, you reference the properties and relationships of your entity beans rather than the underlying tables and columns these objects are mapped to. When an EJQ QL query is executed, the entity manager uses the information you provided through the mapping metadata, discussed in the previous two chapters, and automatically translates it to one (or several) native SQL query. This generated native SQL is then executed through a JDBC driver directly on your database. Since EJB QL is a query language that represents Java objects, it is portable across vendor database implementations because the entity manager handles the conversion to raw SQL for you.
The EJB QL language is easy for developers to learn yet precise enough to be interpreted into native database code. This rich and flexible query language empowers developers while executing in fast native code at runtime. Plus, because EJB QL is object-oriented, queries are usually much more compact and readable than their SQL equivalent. ...
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