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:
packagecom.cndatacom.jpa.entity;
importjava.util.HashSet;
importjava.util.Set;
importjavax.persistence.CascadeType;
importjavax.persistence.Column;
importjavax.persistence.Entity;
importjavax.persistence.FetchType;
importjavax.persistence.GeneratedValue;
importjavax.persistence.Id;
importjavax.persistence.OneToMany;
importjavax.persistence.Table;
@Entity
@Table(name="team")
publicclassTeam{
@Id
@GeneratedValue
privateLong id;
@Column(name="name",length=32)
privateString name;
@OneToMany(mappedBy="team",cascade=CascadeType.ALL,fetch=FetchType.LAZY)
privateSet players =newHashSet();
//以下省略了getter/setter方法
//......
}
Player.java:
packagecom.cndatacom.jpa.entity;
importjavax.persistence.CascadeType;
importjavax.persistence.Column;
importjavax.persistence.Entity;
importjavax.persistence.GeneratedValue;
importjavax.persistence.Id;
importjavax.persistence.JoinColumn;
importjavax.persistence.ManyToOne;
importjavax.persistence.Table;
@Entity
@Table(name="player")
publicclassPlayer{
@Id
@GeneratedValue
privateLong id;
@Column(name="name")
privateString name;
@ManyToOne(cascade={CascadeType.MERGE,CascadeType.REFRESH})
@JoinColumn(name="team_id")
privateTeam 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:
SELECTDISTINCTtFROMTeam tJOINt.players pwherep.nameLIKE:name
或者使用以下语句:
SELECTDISTINCTtFROMTeam t,IN(t.players) pWHEREp.nameLIKE:name
上面两条语句是等价的,产生的SQL语句如下:
select
distinctteam0_.idasid0_,
team0_.nameasname0_
from
team team0_
innerjoin
player players1_
onteam0_.id=players1_.team_id
where
players1_.namelike?
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:
SELECTDISTINCTtFROMTeam tWHEREt.players.nameLIKE: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:
SELECTpFROMPlayer pJOINp.team tWHEREt.id = :id
或者使用以下语句:
SELECTpFROMPlayer p,IN(p.team) tWHEREt.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_.idasid1_,
player0_.nameasname1_,
player0_.team_idasteam3_1_
from
player player0_
innerjoin
team team1_
onplayer0_.team_id=team1_.id
where
team1_.id=?
Hibernate:
select
team0_.idasid2_0_,
team0_.nameasname2_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:
SELECTpFROMPlayer pWHEREp.team.id = :id
这条语句产生的SQL如下:(产生了两条SQL)
Hibernate:
select
player0_.idasid1_,
player0_.nameasname1_,
player0_.team_idasteam3_1_
from
player player0_
where
player0_.team_id=?
Hibernate:
select
team0_.idasid0_0_,
team0_.nameasname0_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:
SELECTpFROMPlayer pJOINFETCHp.team tWHEREt.id = :id
这条查询语句产生的SQL如下:
Hibernate:
select
player0_.idasid1_0_,
team1_.idasid2_1_,
player0_.nameasname1_0_,
player0_.team_idasteam3_1_0_,
team1_.nameasname2_1_
from
player player0_
innerjoin
team team1_
onplayer0_.team_id=team1_.id
where
team1_.id=?