場(chǎng)景
在實(shí)際的開發(fā)過(guò)程中，往往會(huì)從多個(gè)表里獲取數(shù)據(jù)。此時(shí)需要用到多表操作。如查詢用戶的個(gè)人信息及部門信息。

實(shí)例
UserMapper.java

public interface UserMapper {
  UserInfo getUserInfoById (Integer id);
}

DepartmentMapper.java

public interface DepartmentMapper {
  Department getDepartmentByUserId (Integer uid);
}

UserMapper.xml

<select id="getUserInfo" resultMap="userInfoMap">
  SELECT username, age, workno, deptid
  FROM user WHERE uid = #{id}
</select>

<resultMap id="userInfoMap" type="com.cat.pojo.UserInfo">
  <result column="username" property="uname"/>
  <result column="age" property="age"/>
  <result column="workno" property="workNo"/>

  <!-- 將查詢出來(lái)的User對(duì)象的deptid傳入，封裝成具體的Department信息，得到用戶的信息及Department信息 -->
  <association property="dept" column="deptid" select="com.cat.mapper.DepartmentMapper.getDepartmentByUserId" />
</resultMap>

DepartmentMapper.xml

<select id="getDepartmentByUserId" parameterType="string" resultType="com.cat.pojo.Department">
  SELECT department_name, leader FROM department WHERE id = #{deptId}
</select>

SQL執(zhí)行過(guò)程
此時(shí)數(shù)據(jù)的查詢將會(huì)分為兩步，第一步將用戶檢索出來(lái)，第二步再根據(jù)每個(gè)用戶的deptid查詢到部門信息。