Java学习-MyBatis学习(四)

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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">
        
        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>
        
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    settings>

• ③使用resultMap

    <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>
    
    <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>
    
    <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 property="dept" javaType="Dept">
            <id property="did" column="did"/>
            <id property="deptName" column="dept_name"/>
        association>
    resultMap>
    
    <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">
    
    <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"/>
        
        <association property="dept" select="com.lotus.mybatis.mapper.DeptMapper.getEmpAndDeptByStepTwo"
                     column="did" fetchType="eager">association>
    resultMap>
    
    <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>
    
    <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
 @Test
    public 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>
    
    <select id="getDeptAndEmpByStepOne" resultMap="deptAndEmpByStepResultMap">
        select * from t_dept where did=#{did}
    select>


    <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
 @Test
    public void testGetDeptAndEmpStep() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = mapper.getDeptAndEmpByStepOne(1);
        System.out.println(dept);
    }