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小編給大家分享一下如何實(shí)現(xiàn)JPQL的關(guān)聯(lián)查詢,相信大部分人都還不怎么了解,因此分享這篇文章給大家參考一下,希望大家閱讀完這篇文章后大有收獲,下面讓我們一起去了解一下吧!
從一關(guān)聯(lián)到多的查詢和從多關(guān)聯(lián)到一的查詢來(lái)簡(jiǎn)單說(shuō)說(shuō)關(guān)聯(lián)查詢。
實(shí)體Team:球隊(duì)。
實(shí)體Player:球員。
球隊(duì)和球員是一對(duì)多的關(guān)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(duì) * @author Luxh */ @Entity @Table(name="team") public class Team{ @Id @GeneratedValue private Long id; /**球隊(duì)名稱(chēng)*/ @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(duì)*/ @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(guān)聯(lián)到Many的一方:
查找出球員所屬的球隊(duì),可以使用以下語(yǔ)句:
SELECT DISTINCT t FROM Team t JOIN t.players p where p.name LIKE :name
或者使用以下語(yǔ)句:
SELECT DISTINCT t FROM Team t,IN(t.players) p WHERE p.name LIKE :name
上面兩條語(yǔ)句是等價(jià)的,產(chǎn)生的SQL語(yǔ)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(yǔ)句中可以看到team inner join 到player。inner join要求右邊的表達(dá)式必須有返回值。
不能使用以下語(yǔ)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(guān)聯(lián)到One的一方:
查找出某個(gè)球隊(duì)下的所有球員,可以使用以下查詢語(yǔ)句:
SELECT p FROM Player p JOIN p.team t WHERE t.id = :id
或者使用以下語(yǔ)句:
SELECT p FROM Player p, IN(p.team) t WHERE t.id = :id
這兩條查詢語(yǔ)句是等價(jià)的,產(chǎn)生的SQL語(yǔ)句如下:(產(chǎn)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(guān)聯(lián)到One的查詢,還可以使用以下的查詢語(yǔ)句:
SELECT p FROM Player p WHERE p.team.id = :id
這條語(yǔ)句產(chǎn)生的SQL如下:(產(chǎn)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(guān)聯(lián)查詢都產(chǎn)生了兩條SQL,還可以使用join fetch只產(chǎn)生一條SQL語(yǔ)句。查詢語(yǔ)句如下:
SELECT p FROM Player p JOIN FETCH p.team t WHERE t.id = :id
這條查詢語(yǔ)句產(chǎn)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“如何實(shí)現(xiàn)JPQL的關(guān)聯(lián)查詢”這篇文章的所有內(nèi)容,感謝各位的閱讀!相信大家都有了一定的了解,希望分享的內(nèi)容對(duì)大家有所幫助,如果還想學(xué)習(xí)更多知識(shí),歡迎關(guān)注億速云行業(yè)資訊頻道!
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