package com.atguigu.mybatis.pojo;

public class Emp {
    private Integer empId;
    private String empName;
    private Integer age;
    private String gender;
    private Dept dept;

    public Emp() {
    }

    public Emp(Integer empId, String empName, Integer age, String gender, Dept dept) {
        this.empId = empId;
        this.empName = empName;
        this.age = age;
        this.gender = gender;
        this.dept = dept;
    }

    public Integer getEmpId() {
        return empId;
    }

    public void setEmpId(Integer empId) {
        this.empId = empId;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Dept getDept() {
        return dept;
    }

    public void setDept(Dept dept) {
        this.dept = dept;
    }

    @Override
    public String toString() {
        return "Emp{" +
                "empId=" + empId +
                ", empName='" + empName + '\'' +
                ", age=" + age +
                ", gender='" + gender + '\'' +
                ", dept=" + dept +
                '}';
    }
}

实体类：Dept

package com.atguigu.mybatis.pojo;

public class Dept {
    private Integer deptId;
    private String deptName;

    public Dept() {
    }

    public Dept(Integer deptId, String deptName) {
        this.deptId = deptId;
        this.deptName = deptName;
    }

    public Integer getDeptId() {
        return deptId;
    }

    public void setDeptId(Integer deptId) {
        this.deptId = deptId;
    }

    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    @Override
    public String toString() {
        return "Dept{" +
                "deptId=" + deptId +
                ", deptName='" + deptName + '\'' +
                '}';
    }
}

接口：

public interface EmpMapper {
    Emp getEmpById(@Param("empId") Integer empId);
}

SQL:

select * from t_emp where emp_id = #{empId};

测试：

   public void test(){
        SqlSessionUtils sqlSessionUtils = new SqlSessionUtils();
        SqlSession sqlSession = sqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp empById = mapper.getEmpById(1);
        System.out.println(empById.toString());

    }

运行结果：

原因：JAVA中属性名与数据库中字段名不一致

1.使用全局配置处理字段名和属性名不一致的情况

若字段名和实体类中的属性名不一致，但是字段名符合数据库的规则（使用_），实体类中的属性

名符合Java的规则（使用驼峰）

此时也可通过以下两种方式处理字段名和实体类中的属性的映射关系：

1.可以通过为字段起别名的方式，保证和实体类中的属性名保持一致

select emp_id empId,emp_name empName,age,gender from t_emp where emp_id = #{empId};

2.可以在MyBatis的核心配置文件中设置一个全局配置信息mapUnderscoreToCamelCase，可

以在查询表中数据时，自动将_类型的字段名转换为驼峰

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

例如：字段名user_name，设置了mapUnderscoreToCamelCase，此时字段名就会转换为

userName

2.resultMap处理字段和属性的映射关系

1.处理一对一映射

<resultMap id="empResultMap" type="Emp">
    <id property="empId" column="emp_id"></id>
    <result property="empName" column="emp_name"></result>
    <result property="age" column="age"></result>
    <result property="gender" column="gender"></result>
</resultMap>

    <select id="getEmpById" resultMap="empResultMap">
        select * from t_emp where emp_id = #{empId};
    </select>

resultMap：设置自定义映射

属性：
id：表示自定义映射的唯一标识
type：查询的数据要映射的实体类的类型
子标签：
id：设置主键的映射关系
result：设置普通字段的映射关系
association ：设置多对一的映射关系
collection：设置一对多的映射关系
属性：
property：设置映射关系中实体类中的属性名
column：设置映射关系中表中的字段名

2.多对一映射处理

级联方式处理映射关系

  <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>

            <!--部门中的字段dept_id与Emp实体类中的属性dept中的deptId相对应 -->
            <!--部门中的字段dept_name与Emp实体类中的属性dept中的deptName相对应 -->
            <result column="dept_id" property="dept.deptId"></result>
            <result column="dept_name" property="dept.deptName"></result>
 </resultMap>



    <select id="getEmpAndDeptById" resultMap="empAndDeptResultMap">
      SELECT t_emp.*,t_dept.* FROM t_emp
      LEFT JOIN t_dept
      ON t_emp.dept_id=t_dept.dept_id
      where t_emp.emp_id=#{empId}
    </select>

接口：

Emp getEmpAndDeptById(@Param("empId") Integer empId);

测试：

    public void test2(){
        SqlSessionUtils sqlSessionUtils = new SqlSessionUtils();
        SqlSession sqlSession = sqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp empAndDeptById = mapper.getEmpAndDeptById(1);
        System.out.println(empAndDeptById);
        
    }

使用association处理映射关系

  <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>

分步查询

①查询员工信息

    Emp getEmpAndDeptByStep(@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="com.atguigu.mybatis.mapper.DeptMapper.getDeptByStep" column="dept_id">
        </association>

    </resultMap>

    <select id="getEmpAndDeptByStep" resultMap="empAndDeptByStepResultMap">
     select * from t_emp where emp_id=#{empId};
    </select>

②根据员工所对应的部门 id 查询部门信息

    Dept getDeptByStep(@Param("deptId") Integer deptId);

    <select id="getDeptByStep" resultType="com.atguigu.mybatis.pojo.Dept">
        select * from t_dept where dept_id=#{deptId};
    </select>

测试：

    public void test3(){
        SqlSessionUtils sqlSessionUtils = new SqlSessionUtils();
        SqlSession sqlSession = sqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp empAndDeptByStep = mapper.getEmpAndDeptByStep(1);
        System.out.println(empAndDeptByStep);

    }

测试：

    public void test3(){
        SqlSessionUtils sqlSessionUtils = new SqlSessionUtils();
        SqlSession sqlSession = sqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp empAndDeptByStep = mapper.getEmpAndDeptByStep(1);
        System.out.println(empAndDeptByStep.getEmpName());

    }

未开启延迟加载（执行了2次SQL）：

开启延迟加载：

    <settings>
 
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

执行了1次SQL：

3.一对多映射处理

1.collection

接口：

Dept getDeptAndEmpByDeptId(@Param("deptId") Integer deptId);

实体类中，添加 emps集合：

DeptMapper.xml：

    <resultMap id="DeptAndEmpByDeptIdResultMap" 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="DeptAndEmpByDeptIdResultMap">
      SELECT t_emp.*,t_dept.* FROM t_dept
      LEFT JOIN t_emp
      ON t_emp.dept_id=t_dept.dept_id
      WHERE t_dept.dept_id=#{deptId}
    </select>

测试：

    public void test4(){
        SqlSessionUtils sqlSessionUtils = new SqlSessionUtils();
        SqlSession sqlSession = sqlSessionUtils.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept deptAndEmpByDeptId = mapper.getDeptAndEmpByDeptId(1);
        System.out.println(deptAndEmpByDeptId);


    }

运行结果：

2.分步查询

①查询部门信息

    Dept getDeptAndEmpByStepOne(@Param("deptId") Integer deptId);

    <resultMap id="DeptAndEmpByStepResultMap" type="Dept">
        <id column="dept_id" property="deptId"></id>
        <result column="dept_name" property="deptName"></result>
        <collection property="emps"
                    select="com.atguigu.mybatis.mapper.EmpMapper.getDeptAndEmpByStepTwo"
                    column="dept_id">
            
        </collection>

    </resultMap>

    <select id="getDeptAndEmpByStepOne" resultMap="DeptAndEmpByStepResultMap">
        select * from t_dept where dept_id=#{deptId}
    </select>

②根据部门 id 查询部门中的所有员工

    List<Emp> getDeptAndEmpByStepTwo(@Param("deptId") Integer deptId);

    <select id="getDeptAndEmpByStepTwo" resultType="Emp">
         select * from t_emp where dept_id=#{deptId}
    </select>

测试：

    public void test5(){
        SqlSessionUtils sqlSessionUtils = new SqlSessionUtils();
        SqlSession sqlSession = sqlSessionUtils.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept deptAndEmpByStepOne = mapper.getDeptAndEmpByStepOne(1);
        System.out.println(deptAndEmpByStepOne);


    }