Joins, reporting queries, and subselects Joins expressed in (Shared web hosting)
Joins, reporting queries, and subselects Joins expressed in the FROM clause Hibernate differentiates between the purposes for joining. Suppose you re querying Items. There are two possible reasons why you may be interested in joining them with Bids. You may want to limit the item returned by the query on the basis of some criterion that should be applied to their Bids. For example, you may want all Items that have a bid of more than $100; hence this requires an inner join. You aren t interested in items that have no bids so far. On the other hand, you may be primarily interested in the Items, but you may want to execute an outer join just because you want to retrieve all the Bids for the queried Items in the same single SQL statement, something we called eager join fetching earlier. Remember that you prefer to map all associations lazy by default, so an eager, outer-join fetch query is used to override the default fetching strategy at runtime for a particular use case. Let s first write some queries that use inner joins for the purpose of restriction. If you want to retrieve Item instances and restrict the result to items that have bids with a certain amount, you have to assign an alias to a joined association: from Item i join i.bids b where i.description like ‘%Foo%’ and b.amount > 100 This query assigns the alias i to the entity Item and the alias b to the joined Items bids. You then use both aliases to express restriction criteria in the WHERE clause. The resulting SQL is: select i.DESCRIPTION, i.INITIAL_PRICE, … b.BID_ID, b.AMOUNT, b.ITEM_ID, b.CREATED_ON from ITEM i inner join BID b on i.ITEM_ID = b.ITEM_ID where i.DESCRIPTION like ‘%Foo%’ and b.AMOUNT > 100 The query returns all combinations of associated Bids and Items as ordered pairs: Query q = session.createQuery(”from Item i join i.bids b”); Iterator pairs = q.list().iterator(); while ( pairs.hasNext() ) { Object[] pair = (Object[]) pairs.next(); Item item = (Item) pair[0]; Bid bid = (Bid) pair[1]; }
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