qr code java app download Using native SQL queries in Java

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Using native SQL queries
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collection of Object[]s, where in each array index 0 is an Item instance and index 1 is a User instance, can be written as follows:
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em.createNativeQuery("select " + "i.ITEM_ID, i.ITEM_PRICE, u.USERNAME, u.EMAIL " + "from ITEM i join USERS u where i.SELLER_ID = u.USER_ID", "ItemSellerResult");
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The last argument, ItemSellerResult, is the name of a result mapping you define in metadata (at the class or global JPA XML level):
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@SqlResultSetMappings({ @SqlResultSetMapping( name = "ItemSellerResult", entities = { @EntityResult(entityClass = auction.model.Item.class), @EntityResult(entityClass = auction.model.User.class) } ) })
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This resultset mapping likely doesn t work for the query we ve shown remember that for automatic mapping, all columns that are required to instantiate Item and User objects must be returned in the SQL query. It s unlikely that the four columns you return represent the only persistent properties. For the sake of the example, let s assume that they are and that your actual problem is the names of the columns in the resultset, which don t match the names of the mapped columns. First, add aliases to the SQL statement:
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em.createNativeQuery("select " + "i.ITEM_ID as ITEM_ID, i.ITEM_PRICE as ITEM_PRICE, " + "u.USERNAME as USER_NAME, u.EMAIL as USER_EMAIL " + "from ITEM i join USERS u on i.SELLER_ID = u.USER_ID", "ItemSellerResult");
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Next, use @FieldResult in the resultset mapping to map aliases to fields of the entity instances:
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@SqlResultSetMapping( name = "ItemSellerResult", entities = { @EntityResult( entityClass = auction.model.Item.class, fields = { @FieldResult(name = "id", column = "ITEM_ID"), @FieldResult(name = "initialPrice", column = "ITEM_PRICE") }), @EntityResult(
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Advanced query options
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entityClass = auction.model.User.class, fields = { @FieldResult(name = "username", column = "USER_NAME"), @FieldResult(name = "email", column = "USER_EMAIL") }) })
You can also return scalar typed results. The following query returns auction item identifiers and the number of bids for each item:
em.createNativeQuery("select " + "i.ITEM_ID as ITEM_ID, count(b.*) as NUM_OF_BIDS " + "from ITEM i join BIDS b on i.ITEM_ID = b.ITEM_ID " + "group by ITEM_ID", "ItemBidResult");
The resultset mapping doesn t contain entity result mappings this time, only columns:
@SqlResultSetMapping( name = "ItemBidResult", columns = { @ColumnResult(name = "ITEM_ID"), @ColumnResult(name = "NUM_OF_BIDS") })
The result of this query is a collection of Object[]s, with two fields, both of some numeric type (most likely long). If you want to mix entities and scalar types as a query result, combine the entities and columns attributes in a @SqlResultSetMapping. Finally, note that the JPA specification doesn t require that named parameter binding is supported for native SQL queries. Hibernate supports this. Next, we discuss another more exotic but convenient Hibernate feature (Java Persistence doesn t have an equivalent): collection filters.
15.3 Filtering collections
You may wish to execute a query against all elements of a collection. For instance, you may have an Item and wish to retrieve all bids for that particular item, ordered by the time that the bid was created. You can map a sorted or ordered collection for that purpose, but there is an easier choice. You can write a query, and you should already know how:
session.createQuery("from Bid b where b.item = :givenItem" + " order by b.created asc") .setEntity("givenItem", item);
Filtering collections
This query works because the association between bids and items is bidirectional and each Bid knows its Item. There is no join in this query; b.item refers to the ITEM_ID column in the BID table, and you set the value for the comparison directly. Imagine that this association is unidirectional Item has a collection of Bids, but no inverse association exists from Bid to Item. You can try the following query:
select b from Item i join i.bids b where i = :givenItem order by b.amount asc
This query is inefficient it uses an entirely unnecessary join. A better, more elegant solution is to use a collection filter a special query that can be applied to a persistent collection (or array). It s commonly used to further restrict or order a result. You apply it on an already loaded Item and its collection of bids:
List filteredCollection = session.createFilter( item.getBids(), "order by this.created asc" ).list();
This filter is equivalent to the first query of this section and results in identical SQL. The createFilter() method on the Session takes two arguments: a persistent collection (it doesn t have to be initialized) and an HQL query string. Collection filter queries have an implicit FROM clause and an implicit WHERE condition. The alias this refers implicitly to elements of the collection of bids. Hibernate collection filters aren t executed in memory. The collection of bids may be uninitialized when the filter is called and, if so, remains uninitialized. Furthermore, filters don t apply to transient collections or query results. They may be applied only to a persistent collection currently referenced by an entity instance attached to the Hibernate persistence context. The term filter is somewhat misleading, because the result of filtering is a completely new and different collection; the original collection isn t touched. The only required clause of a HQL query is the FROM clause. Because a collection filter has an implicit FROM clause, the following is a valid filter:
List filteredCollection = session.createFilter( item.getBids(), "" ).list();
To the great surprise of everyone, including the designer of this feature, this trivial filter turns out to be useful. You may use it to paginate collection elements:
List filteredCollection = session.createFilter( item.getBids(), "" ) .setFirstResult(50) .setMaxResults(100) .list();
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