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解决字段名与属性名不一致

• ①使用别名emp_name empName解决字段名和属性名不一致

<select id="getAllEmpOld" resultType="Emp"><!--①使用别名emp_name empName解决字段名和属性名不一致-->select eid,emp_name empName,age,sex,email from t_emp;</select>

• ②在全局配置文件中添加全局配置

<settings><!-- 将_自动映射为驼峰，emp_name：empName--><setting name="mapUnderscoreToCamelCase" value="true"/></settings>

• ③使用resultMap

<!--resultMap：设置自定义映射关系id：唯一标识type：设置映射关系中的实体类类型id：设置主键属性：property：设置映射关系中的属性名，必须是type属性所设置的实体类类型中的属性名column：设置映射关系中的字段名，必须是SQL语句查询出的字段名--><resultMap id="empResultMap" type="Emp"><id property="eid" column="eid"/><id property="empName" column="emp_name"/><id property="age" column="age"/><id property="sex" column="sex"/><id property="email" column="email"/></resultMap><!--③使用resultMap解决字段名和属性名不一致--><select id="getAllEmp" resultMap="empResultMap">select * from t_emp;</select>

多对一映射关系

• ①级联属性赋值

 <resultMap id="empAndDeptResultMapOne" type="Emp"><id property="eid" column="eid"/><id property="empName" column="emp_name"/><id property="age" column="age"/><id property="sex" column="sex"/><id property="email" column="email"/><id property="dept.did" column="did"/><id property="dept.deptName" column="dept_name"/></resultMap><!--Emp getEmpAndDept(@Param("eid") Integer eid);--><select id="getEmpAndDept" resultMap="empAndDeptResultMapOne">select * from t_emp left join t_dept on t_emp.did = t_dept.did where t_emp.eid=#{eid}</select>

• ②使用association

<resultMap id="empAndDeptResultMapTwo" type="Emp"><id property="eid" column="eid"/><id property="empName" column="emp_name"/><id property="age" column="age"/><id property="sex" column="sex"/><id property="email" column="email"/><!-- association：处理多对一的映射关系javaType：该属性的类型--><association property="dept" javaType="Dept"><id property="did" column="did"/><id property="deptName" column="dept_name"/></association></resultMap><!--Emp getEmpAndDept(@Param("eid") Integer eid);--><select id="getEmpAndDept" resultMap="empAndDeptResultMapTwo">select * from t_emp left join t_dept on t_emp.did = t_dept.did where t_emp.eid=#{eid}</select>
<mapper namespace="com.lotus.mybatis.mapper.DeptMapper"><!--Dept getEmpAndDeptByStepTwo(@Param("did") Integer did);--><select id="getEmpAndDeptByStepTwo" resultType="Dept">select * from t_dept where did=#{did}</select>
</mapper>

• ③分步查询

<resultMap id="empAndDeptByStepResultMap" type="Emp"><id property="eid" column="eid"/><id property="empName" column="emp_name"/><id property="age" column="age"/><id property="sex" column="sex"/><id property="email" column="email"/><!--select：设置分步查询的SQL唯一标识(namespace,SQLID或mapper接口的全类名.方法名)column：设置分步查询的条件fetchType(eager|lazy)：当开启全局延迟加载后，通过此属性手动控制延迟加载的效果,eager表示立即加载--><association property="dept" select="com.lotus.mybatis.mapper.DeptMapper.getEmpAndDeptByStepTwo"column="did" fetchType="eager"></association></resultMap><!-- Emp getEmpAndDeptByStepOne(); --><select id="getEmpAndDeptByStepOne" resultMap="empAndDeptByStepResultMap">select * from t_emp where eid=#{eid}</select>

public interface EmpMapper {
/*** 通过分步查询查询员工以及员工所对应部门信息* 第一步，查询员工信息*/Emp getEmpAndDeptByStepOne(@Param("eid") Integer eid);}public interface DeptMapper {/*** 通过分步查询查询员工以及员工所对应部门信息* 第一步，通过did查询员工对应的部门信息*/Dept getEmpAndDeptByStepTwo(@Param("did") Integer did);
}

一对多映射关系

• ①使用collection标签

 <resultMap id="deptAndEmpResultMap" type="Dept"><id property="did" column="did"/><id property="deptName" column="dept_name"/><collection property="emps" ofType="Emp"><id property="eid" column="eid"/><id property="empName" column="emp_name"/><id property="age" column="age"/><id property="sex" column="sex"/><id property="email" column="email"/></collection></resultMap><!--Dept getDeptAndEmp(@Param("did") Integer did);--><select id="getDeptAndEmp" resultMap="deptAndEmpResultMap">select * from t_dept left join t_emp on t_dept.did = t_emp.did where t_dept.did=#{did}</select>

/*** 获取部门及部门中所有员工信息*/Dept getDeptAndEmp(@Param("did") Integer did);
//测试代码@Testpublic void testGetDeptAndEmp() {SqlSession sqlSession = SqlSessionUtils.getSqlSession();DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);Dept dept = mapper.getDeptAndEmp(1);System.out.println(dept);}

• ②使用分步查询

<!---DeptMapper.xml>
<resultMap id="deptAndEmpByStepResultMap" type="Dept"><id property="did" column="did"/><result property="deptName" column="dept_name"/><collection property="emps"select="com.lotus.mybatis.mapper.EmpMapper.getDeptAndEmpByStepTwo"column="did"></collection></resultMap><!--Dept getDeptAndEmpByStepOne(@Param("did") Integer did);--><select id="getDeptAndEmpByStepOne" resultMap="deptAndEmpByStepResultMap">select * from t_dept where did=#{did}</select>
<!-- EmpMapper.xml -->
<!--List<Emp> getDeptAndEmpByStepTwo(@Param("did") Integer did);--><select id="getDeptAndEmpByStepTwo" resultType="Emp">select * from t_emp where did=#{did}</select>

//----DeptMapper
/*** 分步查询①查询部门信息*/Dept getDeptAndEmpByStepOne(@Param("did") Integer did);
//----EmpMapper
/*** 分步查询②根据did查询员工信息*/
List<Emp> getDeptAndEmpByStepTwo(@Param("did") Integer did);//测试代码@Testpublic void testGetDeptAndEmpStep() {SqlSession sqlSession = SqlSessionUtils.getSqlSession();DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);Dept dept = mapper.getDeptAndEmpByStepOne(1);System.out.println(dept);}