Mybatis关联查询

1.一对一查询

员工表pojo

@Data
@Accessors(chain = true)
@NoArgsConstructor
@AllArgsConstructor
public class Emp implements Serializable {
    private Integer id;
    private String name;
    private Integer age;
//    private Integer deptId;
    private Dept dept;//一个员工对应一个部门，所以使用对象封装
}

部门表pojo

@Data
@NoArgsConstructor
@AllArgsConstructor
@Accessors(chain = true)
public class Dept implements Serializable {
    private Integer deptId;
    private String deptName;
    private List<Emp> emps;//一个部门有多个员工，所以使用list封装
}

	**查询员工表emp所对应的部门表dept。两种方式：**

1.1单条sql语句查询

<select id="findAll" resultMap="empRM">
    select emp.id,emp.name,emp.age,emp.dept_id,dept.dept_name
    from emp join dept on emp.dept_id=dept.dept_id
</select>
    <resultMap id="empRM" type="Emp" autoMapping="true">
        <id column="id" property="id"></id>
        <association property="dept" javaType="Dept" >
            <id column="dept_id" property="deptId"/>
            <result column="dept_name" property="deptName"/>
        </association>
    </resultMap>

1.2多条语句查询

<select id="findAllWhere" resultMap="empRM2">
        select * from emp
    </select>

    <resultMap id="empRM2" type="Emp" autoMapping="true">
        <id column="id" property="id"/>
        <association property="dept" javaType="Dept"
                     column="dept_id" select="findDept"/>
    </resultMap>

    <select id="findDept" resultMap="deptRM">
        select * from dept where dept_id=#{dept_id}
    </select>

    <resultMap id="deptRM" type="Dept">
        <id column="dept_id" property="deptId"/>
        <result column="dept_name" property="deptName"/>
    </resultMap>