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You ve significantly simplified the code again. The nicest thing about Hibernate Example queries is that an Example is just an ordinary Criterion. You can freely mix and match query by example with query by criteria. Let s see how this works by further restricting the search results to users with unsold Items. For this purpose, you may add a Criteria to the example user, constraining the result using its items collection of Items:
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public List findUsersByExample(User u){ Example exampleUser = Example.create(u) .ignoreCase() .enableLike(MatchMode.ANYWHERE); return getSession().createCriteria(User.class) .add( exampleUser ) .createCriteria("items") .add( Restrictions.isNull("successfulBid") ) .list(); }
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Even better, you can combine User properties and Item properties in the same search:
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public List findUsersByExample(User u, Item i) { Example exampleUser = Example.create(u).ignoreCase().enableLike(MatchMode.ANYWHERE); Example exampleItem = Example.create(i).ignoreCase().enableLike(MatchMode.ANYWHERE); return getSession().createCriteria(User.class) .add( exampleUser ) .createCriteria("items") .add( exampleItem ) .list(); }
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At this point, we invite you to take a step back and consider how much code would be required to implement this search screen using hand-coded SQL/JDBC. We won t reproduce it here; it would stretch for pages. Also note that the client of the findUsersByExample() method doesn t need to know anything about Hibernate, and it can still create complex criteria for searching. If HQL, JPA QL, and even Criteria and Example aren t powerful enough to express a particular query, you must fall back to native SQL.
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Using native SQL queries
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15.2 Using native SQL queries
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HQL, JPA QL, or criteria queries should be flexible enough to execute almost any
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query you like. They refer to the mapped object schema; hence, if your mapping works as expected, Hibernate s queries should give you the power you need to retrieve data any way you like. There are a few exceptions. If you want to include a native SQL hint to instruct the database management systems query optimizer, for example, you need to write the SQL yourself. HQL, JPA QL, and criteria queries don t have keywords for this. On the other hand, instead of falling back to a manual SQL query, you can always try to extend the built-in query mechanisms and include support for your special operation. This is more difficult to do with HQL and JPA QL, because you have to modify the grammar of these string-based languages. It s easy to extend the Criteria API and add new methods or new Criterion classes. Look at the Hibernate source code in the org.hibernate.criterion package; it s well designed and documented. When you can t extend the built-in query facilities or prevent nonportable manually written SQL, you should first consider using Hibernate s native SQL query options, which we now present. Keep in mind that you can always fall back to a plain JDBC Connection and prepare any SQL statement yourself. Hibernate s SQL options allow you to embed SQL statements in a Hibernate API and to benefit from extra services that make your life easier. Most important, Hibernate can handle the resultset of your SQL query.
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15.2.1 Automatic resultset handling
The biggest advantage of executing an SQL statement with the Hibernate API is automatic marshaling of the tabular resultset into business objects. The following SQL query returns a collection of Category objects:
List result = session.createSQLQuery("select * from CATEGORY") .addEntity(Category.class) .list();
Hibernate reads the resultset of the SQL query and tries to discover the column names and types as defined in your mapping metadata. If the column CATEGORY_ NAME is returned, and it s mapped to the name property of the Category class, Hibernate knows how to populate that property and finally returns fully loaded business objects.
Advanced query options
The * in the SQL query projects all selected columns in the resultset. The automatic discovery mechanism therefore works only for trivial queries; more complex queries need an explicit projection. The next query returns a collection of Item objects:
session.createSQLQuery("select {i.*} from ITEM i" + " join USERS u on i.SELLER_ID = u.USER_ID" + " where u.USERNAME = :uname") .addEntity("i", Item.class) .setParameter("uname", "johndoe");
The SQL SELECT clause includes a placeholder which names the table alias i and projects all columns of this table into the result. Any other table alias, such as the joined USERS table, which is only relevant for the restriction, isn t included in the resultset. You now tell Hibernate with addEntity() that the placeholder for alias i refers to all columns that are needed to populate the Item entity class. The column names and types are again automatically guessed by Hibernate during query execution and result marshaling. You can even eagerly fetch associated objects and collections in a native SQL query:
session.createSQLQuery("select {i.*}, {u.*} from ITEM i" + " join USERS u on i.SELLER_ID = u.USER_ID" + " where u.USERNAME = :uname") .addEntity("i", Item.class) .addJoin("u", "i.seller") .setParameter("uname", "johndoe");
This SQL query projects two sets of columns from two table aliases, and you use two placeholders. The i placeholder again refers to the columns that populate the Item entity objects returned by this query. The addJoin() method tells Hibernate that the u alias refers to columns that can be used to immediately populate the associated seller of each Item. Automatic marshaling of resultsets into business objects isn t the only benefit of the native SQL query feature in Hibernate. You can even use it if all you want to retrieve is a simple scalar value.
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