JPA的查询语言—JPQL的关联查询

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:

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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:

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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 //...... }

　　1、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：

1	SELECT DISTINCT t FROM Team t JOIN t.players p where p. name LIKE : name

　　或者使用以下语句：

1	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：

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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：

1	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。

　　2、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：

1	SELECT p FROM Player p JOIN p.team t WHERE t.id = :id

　　或者使用以下语句：

1	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)

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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：

1	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)

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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：

1	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：

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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=?