JPA的查詢語言—JPQL的關聯查詢

　　從一關聯到多的查詢和從多關聯到一的查詢來簡單說說關聯查詢。

　　實體Team：球隊。

　　實體Player：球員。

　　球隊和球員是一對多的關系。

　　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;


/**
 * 球隊
 * @author Luxh
 */
@Entity
@Table(name="team")
public class Team{

	@Id
	@GeneratedValue
	private Long id;
	
	/**球隊名稱*/
	@Column(name="name",length=32)
	private String name;
	
	/**擁有的球員*/
	@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;


/**
 * 球員
 * @author Luxh
 */
@Entity
@Table(name="player")
public class Player{
	
	@Id
	@GeneratedValue
	private Long id;
	
	/**球員姓名*/
	@Column(name="name")
	private String name;
	
	/**所屬球隊*/
	@ManyToOne(cascade={CascadeType.MERGE,CascadeType.REFRESH})
	@JoinColumn(name="team_id")
	private Team team;
	
	//以下省略了getter/setter方法

        //......
}

---

　　1、從One的一方關聯到Many的一方：

　　查找出球員所屬的球隊，可以使用以下語句：

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

　　或者使用以下語句：

SELECT DISTINCT t FROM Team t,IN(t.players) p WHERE p.name LIKE :name

　　上面兩條語句是等價的，產生的SQL語句如下：

    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 ?

　　從SQL語句中可以看到team inner join 到player。inner join要求右邊的表達式必須有返回值。

　　不能使用以下語句：

SELECT DISTINCT t FROM Team t  WHERE t.players.name LIKE :name

　　不能使用t.players.name這樣的方式從集合中取值，要使用join或者in才行。

　　2、從Many的一方關聯到One的一方：

　　查找出某個球隊下的所有球員，可以使用以下查詢語句：

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

　　或者使用以下語句：

SELECT p FROM Player p, IN(p.team) t WHERE t.id = :id

　　這兩條查詢語句是等價的，產生的SQL語句如下：(產生了兩條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=?

　　從Many關聯到One的查詢，還可以使用以下的查詢語句：

SELECT p FROM Player p WHERE p.team.id = :id

　　這條語句產生的SQL如下：(產生了兩條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

　　

　　以上從Many到One的關聯查詢都產生了兩條SQL，還可以使用join fetch只產生一條SQL語句。查詢語句如下：

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

　　這條查詢語句產生的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=?