Creating and running queries Queries are the most (Best web site)
Creating and running queries Queries are the most interesting part of writing good data access code. A complex query may require a long time to get right, and its impact on the performance of the application can be tremendous. On the other hand, writing queries becomes much easier with more experience, and what seemed difficult at first is only a matter of knowing some of the more advanced features. If you ve been using handwritten SQL for a number of years, you may be con cerned that ORM will take away some of the expressiveness and flexibility that you re used to. This isn t the case with Hibernate and Java Persistence. Hibernate s powerful query facilities allow you to express almost everything you commonly (or even uncommonly) need to express in SQL, but in object-ori ented terms using classes and properties of classes. We ll show you the differences between native Hibernate queries and the standardized subset in Java Persistence. You may also use this chapter as a reference; hence some sections are written in a less verbose style but show many small code examples for different use cases. We also sometimes skip optimizations in the CaveatEmptor application for better readability. For example, instead of referring to the MonetaryAmount value type, we use a BigDecimal amount in comparisons. First, we show you how queries are executed. Don t let yourself be distracted by the queries; we discuss them soon. 14.1 Creating and running queries Let s start with a few examples so you understand the basic usage. In earlier chapters, we mentioned that there are three ways to express queries in Hibernate: Hibernate Query Language (HQL), and the subset standardized as JPA QL: session.createQuery(”from Category c where c.name like ‘Laptop%’”); entityManager.createQuery( “select c from Category c where c.name like ‘Laptop%’” ); Criteria API for query by criteria (QBC) and query by example (QBE): session.createCriteria(Category.class) .add( Restrictions.like(”name”, “Laptop%”) ); Direct SQL with or without automatic mapping of resultsets to objects: session.createSQLQuery( “select {c.*} from CATEGORY {c} where NAME like ‘Laptop%’” ).addEntity(”c”, Category.class); A query must be prepared in application code before execution. So, querying involves several distinct steps:
Searching for affordable and reliable webhost to host and run your web applications? Go to our java web server services and you will be pleased.