1.1.1. 嵌套查询
使用association结点的select属性可以建立嵌套查询。前文介绍的不使用select的方法在主表结果记录集包含N条记录，每个记录对应1条子表记录的情况下，总共执行一条SQL语句就可以满足整个查询操作的要求，而本文介绍的select属性建立嵌套查询的方法，则
需要执行1+N条SQL语句(单向关联)，或者1+N*2条语句(双向关联)。
Mapper配置：
<!-- 主表resultMap:Person和IdCard的一对一关联:双向关联，嵌套查询 -->
<resultMap type="com.test.mybatis3.pojo.Person" id="personAndIdCardEmbeddedResultMap">
<id property="id" column="id" />
<result property="name" column="name" />
<result property="status" column="status" />
<!-- Person通过idCard属性导航到IdCard -->
<association property="idCard" javaType="com.test.mybatis3.pojo.IdCard" column="id" select="findIdCardByCardId">
</association>
</resultMap>
<!-- 子表resultMap:IdCard的映射 -->
<resultMap type="com.test.mybatis3.pojo.IdCard" id="idCardResultMap">
<id property="cardid" column="cardid" />
<result property="personid" column="personid" />
<!-- IdCard通过person属性导航到Person -->
<association property="person" column="personid" select="findPersonByPersonId">
</association>
</resultMap>
<!-- 主表：查找 Person和对应的IdCard:双向关联-->
<select id="findPersonAndIdCardEmbeded" resultMap="personAndIdCardEmbeddedResultMap">
select * from t_person
</select>
<!-- 子表:通过personid查找IdCard -->
<select id="findIdCardByCardId" parameterType="string" resultMap="idCardResultMap">
select * from t_idcard where personid=#{personid}
</select>
<!-- 用于IdCard的映射中从personid查找Person -->
<select id="findPersonByPersonId" parameterType="string" resultType="com.test.mybatis3.pojo.Person" >
select * from t_person where id=#{id}
</select>
Mapper接口：
/** * 嵌套查询：Person和IdCard. * @return * @throws Exception */ List<Person> findPersonAndIdCardEmbeded() throws Exception;
单元测试代码：
//打开Session。
session = sessionBuilder.openSession();
//找到MyBatis自动实现的PersonMapper接口的代理对象。
PersonMapper personMapper = session.getMapper(PersonMapper.class);
//嵌套查询。
List<Person> persons = personMapper.findPersonAndIdCardEmbeded();
//输出查询结果。
for(Person person : persons){
System.out.println(person);
System.out.println(person.getIdCard());
//查看person和idCard的person是否同一个对象。
System.out.println(person == person.getIdCard().getPerson());
}
运行结果如下：
1 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndIdCardEmbeded - ==> Preparing: select * from t_person
43 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndIdCardEmbeded - ==> Parameters:
67 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndIdCardEmbeded - <== Columns: id, name, status
67 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndIdCardEmbeded - <== Row: lisi, li si, 0
70 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - ====> Preparing: select * from t_idcard where personid=?
70 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - ====> Parameters: lisi(String)
71 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - <==== Columns: personid, cardid
72 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - <==== Row: lisi, 222222222222222222
72 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - ======> Preparing: select * from t_person where id=?
72 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - ======> Parameters: lisi(String)
72 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - <====== Columns: id, name, status
73 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - <====== Row: lisi, li si, 0
74 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - <====== Total: 1
74 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - <==== Total: 1
74 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndIdCardEmbeded - <== Row: zhangsan, zhang san, 0
75 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - ====> Preparing: select * from t_idcard where personid=?
75 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - ====> Parameters: zhangsan(String)
75 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - <==== Columns: personid, cardid
77 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - <==== Row: zhangsan, 1111111111111111
77 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - ======> Preparing: select * from t_person where id=?
78 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - ======> Parameters: zhangsan(String)
78 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - <====== Columns: id, name, status
78 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - <====== Row: zhangsan, zhang san, 0
78 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonByPersonId - <====== Total: 1
79 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findIdCardByCardId - <==== Total: 1
79 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndIdCardEmbeded - <== Total: 2
Person [id=lisi, name=li si, status=0,address=null]
IdCard [cardid=222222222222222222, personid=lisi, person=Person [id=lisi, name=li si, status=0,address=null]]
false
Person [id=zhangsan, name=zhang san, status=0,address=null]
IdCard [cardid=1111111111111111, personid=zhangsan, person=Person [id=zhangsan, name=zhang san, status=0,address=null]]
false
观察运行结果可以发现，在双向关联的情况下，主表2条记录(N=2)，总共执行了1+N+N=5条SQL语句。如果换成让MyBatis做单向关联，在Service方法中编写代码实现反向关联，则只需要执行1+N条SQL语句。