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Employee和Department都是实体类
<resultMap type="Employee" id="result1">
<id column="id" property="id" />
<result column="last_name" property="lastName" />
<result column="email" property="email" />
<result column="gender" property="gender" />
<!-- 1.联合查询（使用对象进行关联） -->
<!-- <result column="dept_id" property="dept.id"/>
<result column="dname" property="dept.dname"/> -->
<!-- 2.关联查询,可以关联一个独立的对象 -->
<association property="dept"
javaType="com.hwua.entity.Department">
<id column="dept_id" property="id" />
<result column="dname" property="dname" />
</association>
</resultMap>
<select id="findById" parameterType="Long" resultMap="result1">
select e.id,e.last_name,e.email,e.gender,e.dept_id,d.dname
from t_emps e,t_dept d where e.dept_id=d.id and e.id=#{id}
</select>
<!-- 分步查询 -->
<resultMap type="Employee" id="result2">
<id column="id" property="id" />
<result column="last_name" property="lastName" />
<result column="email" property="email" />
<result column="gender" property="gender" />
<!-- 3.使用association实现分段查询（分步），将一条sql拆分
property 是pojo中定义的以类为属性的名称：private Department dept;
select可以调用一个接口的方法
column 表示入参,这个接口需要传入的参数 -->
<association property="dept"
select="com.hwua.dao.DeptMapper.findById"
column="dept_id">
<id column="id" property="id" />
<result column="dname" property="dname" />
</association>
</resultMap>
<select id="findByIdStep" parameterType="Long"
resultMap="result2">
select e.id,e.last_name,e.email,e.gender,e.dept_id from
t_emps e where e.id=#{id}
</select>
<resultMap type="com.hwua.entity.Department" id="result1">
<id column="id" property="id" />
<result column="dname" property="dname" />
<!-- 员工信息 collection:封装集合对象
ofType:专门封装集合对象时，用来指定对象类型，不能用javaType -->
<collection property="emps"
ofType="com.hwua.entity.Employee">
<id column="eid" property="id" />
<result column="last_name" property="lastName" />
<result column="email" property="email" />
<result column="gender" property="gender" />
</collection>
</resultMap>
<select id="findFullById" parameterType="Long"
resultMap="result1">
select d.id,dname,e.id eid,e.last_name,e.email,e.gender
from t_dept d LEFT JOIN t_emps e
on e.dept_id=d.id
where d.id=#{1}
</select>
<!-- 分步查询 -->
<resultMap type="com.hwua.entity.Department" id="result2">
<id column="id" property="id" />
<result column="dname" property="dname" />
<collection property="emps"
select="com.hwua.dao.EmpMapper.findByDeptId" column="id">
<id column="id" property="id" />
<result column="last_name" property="lastName" />
<result column="email" property="email" />
<result column="gender" property="gender" />
</collection>
</resultMap>
<select id="findFullByIdStep" parameterType="Long"
resultMap="result2">
select d.id,dname from t_dept d where d.id=#{1}
</select>
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