t_emp
t_dept
员工和部门之间是多对一的关系,而部门和员工是一对多的关系
//Emp
private Integer empId;
private String empName;
private Integer age;
private String gender;
private Dept dept;
//Dept
private Integer deptId;
private String deptName;
private List<Emp> emps;
下面分别演示mybatis中这两种情况的映射的处理
接口抽象方法:Emp getEmpAndDeptByEmpId(@Param("empId") Integer empId);
<resultMap id="empAndDeptResultMap" type="Emp">
<id column="emp_id" property="empId">id>
<result column="emp_name" property="empName">result>
<result column="age" property="age">result>
<result column="gender" property="gender">result>
<result column="dept_id" property="dept.deptId">result>
<result column="dept_name" property="dept.deptName">result>
resultMap>
<select id="getEmpAndDeptByEmpId" resultMap="empAndDeptResultMap">
SELECT * FROM t_emp
LEFT JOIN t_dept
ON t_emp.dept_id = t_dept.dept_id
WHERE emp_id = #{empId};
select>
<resultMap id="empAndDeptResultMap" type="Emp">
<id column="emp_id" property="empId">id>
<result column="emp_name" property="empName">result>
<result column="age" property="age">result>
<result column="gender" property="gender">result>
<association property="dept" javaType="Dept">
<id column="dept_id" property="deptId">id>
<result column="dept_name" property="deptName">result>
association>
resultMap>
<select id="getEmpAndDeptByEmpId" resultMap="empAndDeptResultMap">
SELECT * FROM t_emp
LEFT JOIN t_dept
ON t_emp.dept_id = t_dept.dept_id
WHERE emp_id = #{empId};
select>
EmpMapper
接口抽象方法:
Emp getEmpAndDeptByStepOne(@Param("empId") Integer empId);
<resultMap id="empAndDeptByStepResultMap" type="Emp">
<id column="emp_id" property="empId">id>
<result column="emp_name" property="empName">result>
<result column="age" property="age">result>
<result column="gender" property="gender">result>
<association property="dept"
select="lrc.mapper.DeptMapper.getEmpAndDeptByStepTwo"
column="dept_id">association>
resultMap>
<select id="getEmpAndDeptByStepOne" resultMap="empAndDeptByStepResultMap">
SELECT * FROM t_emp WHERE emp_id = #{empId};
select>
DeptMapper
接口抽象方法:
Dept getEmpAndDeptByStepTwo(@Param("deptId") Integer deptId);
<select id="getEmpAndDeptByStepTwo" resultType="Dept">
SELECT * FROM t_dept WHERE dept_id = #{deptId};
select>
接口抽象方法:Dept getDeptAndEmpByDeptId(@Param("deptId") Integer deptId);
<resultMap id="deptAndEmpResultMap" type="Dept">
<id column="dept_id" property="deptId">id>
<result column="dept_name" property="deptName">result>
<collection property="emps" ofType="Emp">
<id column="emp_id" property="empId">id>
<result column="emp_name" property="empName">result>
<result column="age" property="age">result>
<result column="gender" property="gender">result>
collection>
resultMap>
<select id="getDeptAndEmpByDeptId" resultMap="deptAndEmpResultMap">
SELECT * FROM
t_dept LEFT JOIN t_emp
ON t_dept.dept_id = t_emp.dept_id
WHERE t_dept.dept_id = 2;
select>
DeptMapper
接口抽象方法:
Dept getDeptAndEmpByStepOne(@Param("deptId") Integer deptId);
<resultMap id="empAndDeptResultMap" type="Dept">
<id column="dept_id" property="deptId">id>
<result column="dept_name" property="deptName">result>
<association property="emps"
select="lrc.mapper.EmpMapper.getDeptAndEmpByStepTwo"
column="dept_id">association>
resultMap>
<select id="getDeptAndEmpByStepOne" resultMap="empAndDeptResultMap">
SELECT * FROM t_dept WHERE dept_id = #{deptId};
select>
EmpMapper
接口抽象方法:
List
<select id="getDeptAndEmpByStepTwo" resultType="Emp">
SELECT * FROM t_emp WHERE dept_id = #{deptId};
select>
分步查询的优点:可以实现延迟加载,但是必须在核心配置文件中设置全局配置信息:
lazyLoadingEnabled
:延迟加载的全局开关。当开启时,所有关联对象都会延迟加载
aggressiveLazyLoading
:当开启时,任何方法的调用都会加载该对象的所有属性。否则,每个属性会按需加载
此时就可以实现按需加载,获取的数据是什么,就只会执行相应的sql。
可通过association和collection中的
fetchType
属性设置当前的分步查询是否使用延迟加载,
fetchType="lazy(延迟加载)|eager(立即加载)"