小编给大家分享一下如何实现JPQL的关联查询，相信大部分人都还不怎么了解，因此分享这篇文章给大家参考一下，希望大家阅读完这篇文章后大有收获，下面让我们一起去了解一下吧！
从一关联到多的查询和从多关联到一的查询来简单说说关联查询。
实体Team：球队。
实体Player：球员。
球队和球员是一对多的关系。
Team.java:
package com.cndatacom.jpa.entity; import java.util.HashSet; import java.util.Set; import javax.persistence.CascadeType; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.FetchType; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.OneToMany; import javax.persistence.Table; @Entity @Table(name="team") public class Team{ @Id @GeneratedValue private Long id; @Column(name="name",length=32) private String name; @OneToMany(mappedBy="team",cascade=CascadeType.ALL,fetch=FetchType.LAZY) private Set<Player> players = new HashSet<Player>(); //以下省略了getter/setter方法 //...... }Player.java:
package com.cndatacom.jpa.entity; import javax.persistence.CascadeType; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.ManyToOne; import javax.persistence.Table; @Entity @Table(name="player") public class Player{ @Id @GeneratedValue private Long id; @Column(name="name") private String name; @ManyToOne(cascade={CascadeType.MERGE,CascadeType.REFRESH}) @JoinColumn(name="team_id") private Team team; //以下省略了getter/setter方法 //...... }从One的一方关联到Many的一方：
查找出球员所属的球队，可以使用以下语句：
SELECT DISTINCT t FROM Team t JOIN t.players p where p.name LIKE :name或者使用以下语句：
SELECT DISTINCT t FROM Team t,IN(t.players) p WHERE p.name LIKE :name上面两条语句是等价的，产生的SQL语句如下：
select distinct team0_.id as id0_, team0_.name as name0_ from team team0_ inner join player players1_ on team0_.id=players1_.team_id where players1_.name like ?从SQL语句中可以看到team inner join 到player。inner join要求右边的表达式必须有返回值。
不能使用以下语句：
SELECT DISTINCT t FROM Team t WHERE t.players.name LIKE :name不能使用t.players.name这样的方式从集合中取值，要使用join或者in才行。
从Many的一方关联到One的一方：
查找出某个球队下的所有球员，可以使用以下查询语句：
SELECT p FROM Player p JOIN p.team t WHERE t.id = :id或者使用以下语句：
SELECT p FROM Player p, IN(p.team) t WHERE t.id = :id这两条查询语句是等价的，产生的SQL语句如下：(产生了两条SQL)
Hibernate: select player0_.id as id1_, player0_.name as name1_, player0_.team_id as team3_1_ from player player0_ inner join team team1_ on player0_.team_id=team1_.id where team1_.id=? Hibernate: select team0_.id as id2_0_, team0_.name as name2_0_ from team team0_ where team0_.id=?从Many关联到One的查询，还可以使用以下的查询语句：
SELECT p FROM Player p WHERE p.team.id = :id这条语句产生的SQL如下：(产生了两条SQL)
Hibernate: select player0_.id as id1_, player0_.name as name1_, player0_.team_id as team3_1_ from player player0_ where player0_.team_id=? Hibernate: select team0_.id as id0_0_, team0_.name as name0_0_ from team team0以上从Many到One的关联查询都产生了两条SQL，还可以使用join fetch只产生一条SQL语句。查询语句如下：
SELECT p FROM Player p JOIN FETCH p.team t WHERE t.id = :id这条查询语句产生的SQL如下：
Hibernate: select player0_.id as id1_0_, team1_.id as id2_1_, player0_.name as name1_0_, player0_.team_id as team3_1_0_, team1_.name as name2_1_ from player player0_ inner join team team1_ on player0_.team_id=team1_.id where team1_.id=?以上是“如何实现JPQL的关联查询”这篇文章的所有内容，感谢各位的阅读！相信大家都有了一定的了解，希望分享的内容对大家有所帮助，如果还想学习更多知识，欢迎关注编程网行业资讯频道！
