文章轉載來自：http://www.cnblogs.com/luxh/archive/2012/06/02/2531750.html
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係。
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;/** *
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:
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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;/** *
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：
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SELECT DISTINCT tFROM Team
t JOIN t.players
p where p.name LIKE :name |
或者使用以下語句：
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SELECT DISTINCT tFROM 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：
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select distinct team0_.idas id0_, team0_.name as name0_from team
team0_ inner join player
players1_ on team0_.id=players1_.team_idwhere 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：
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SELECT DISTINCT tFROM 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：
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SELECT pFROM Player
p JOIN p.team
t WHERE t.id
= :id |
或者使用以下語句：
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SELECT pFROM 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)
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Hibernate: select player0_.idas id1_, player0_.name as name1_, player0_.team_idas team3_1_ from player
player0_ inner join team
team1_ on player0_.team_id=team1_.id where team1_.id=?Hibernate: select team0_.idas 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：
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SELECT pFROM Player
p WHERE p.team.id
= :id |
這條語句產生的SQL如下：(產生了兩條SQL)
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Hibernate: select player0_.idas id1_, player0_.name as name1_, player0_.team_idas team3_1_ from player
player0_ where player0_.team_id=?Hibernate: select team0_.idas 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：
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SELECT pFROM Player
p JOIN FETCH p.team
t WHERE t.id
= :id |
這條查詢語句產生的SQL如下：
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Hibernate: select player0_.idas id1_0_, team1_.idas id2_1_, player0_.name as name1_0_, player0_.team_idas team3_1_0_, team1_.name as name2_1_ from player
player0_ inner join team
team1_ on player0_.team_id=team1_.id where team1_.id=? |