SpringBoot进行多表查询功能的实现

实体类：

Emp 类：

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Emp {
    private int id;
    private String lastname;
    private String email;
    private int gender;
    private int did;
    private Dept dept;
    private Date birth = new Date();
}

Dept类：

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Dept {
    private int id;
    private String dname;
}

Mapper接口

EmpMapper:

//这个注解表示这是一个mybatis的mapper类
@Mapper
@Repository
public interface EmpMapper {
    void addEmp(Emp emp);
    void deleteEmp(int id);
    void updateEmp(Emp emp);
    Emp queryEmpById(int id);
    List<Emp> queryEmpList();
}

DeptMapper:

@Mapper
@Repository
public interface DeptMapper {
    List<Dept> queryDeptList(@Param("cid") int cid);
}

EmpMapper.xml 配置文件

<mapper namespace="com.acoffee.mapper.EmpMapper">

    <resultMap id="EmpMap" type="Emp">
        <id column="id" property="id"></id>
        <id column="lastname" property="lastname"></id>
        <id column="email" property="email"></id>
        <id column="gender" property="gender"></id>
        <association property="dept" select="com.acoffee.mapper.DeptMapper.queryDeptList" column="did"></association>
    </resultMap>

    <select id="queryEmpList" resultMap="EmpMap">
        select * from emp_dept.employees
    </select>

</mapper>

DeptMapper.xml配置文件

<mapper namespace="com.acoffee.mapper.DeptMapper">

    <select id="queryDeptList" resultType="Dept">
        select * from emp_dept.department where id = #{id};
    </select>
    
</mapper>

前端页面（部分）

<tr th:each="emp:${emps}">
	  <td th:text="${emp.getId()}"></td>
	  <td th:text="${emp.getLastname()}"></td>
	  <td th:text="${emp.getEmail()}"></td>
	  <td th:text="${emp.getGender()==0?"女":"男"}"></td>
	  <td th:text="${emp.dept.getDname()}"></td>
	  <td th:text="${#dates.format(emp.getBirth(),"yyyy-MM-dd HH:mm:ss")}"></td>
	  <td>
	      <a class="btn btn-sm btn-primary" th:href="@{/emp/{id}/(id=${emp.getId()})}" rel="external nofollow" >编辑</a>
	      <a class="btn btn-sm btn-danger" th:href="@{/delemp/{id}/(id=${emp.getId()})}" rel="external nofollow" >删除</a>
	  </td>
</tr>

查询结果：

SpringBoot进行多表查询功能的实现

上述我们采用的是分步查询。

我们下面使用association嵌套映射

其实这里查询会出现一个奇怪的事情，

SpringBoot进行多表查询功能的实现

因为我们现在两个表中都有id这个字段，所以我们在映射时使用<result property="id" column="id"></result>去查找部门的id时就发现查出来的是员工的id，就是因为员工的id与部门的id字段名重名了？

EmpMapper:

<mapper namespace="com.acoffee.mapper.EmpMapper">

    <resultMap id="EmpMap" type="Emp">
        <id column="id" property="id"></id>
        <id column="lastname" property="lastname"></id>
        <id column="email" property="email"></id>
        <id column="gender" property="gender"></id>
        <association property="dept" javaType="com.acoffee.pojo.Dept">
            <result property="id" column="id"></result>
            <result property="dname" column="dname"></result>
        </association>
    </resultMap>

    <select id="queryEmpList" resultMap="EmpMap">
        select emp.*,dept.* from emp_dept.employees emp,emp_dept.department dept where emp.did = dept.id
    </select>
</mapper>

上述执行结果如下

SpringBoot进行多表查询功能的实现

我们发现是员工id，我们现在将数据库中部门表的id改为pid

SpringBoot进行多表查询功能的实现

修改配置文件以及实体类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Dept {
    private int pid;
    private String dname;
}

<mapper namespace="com.acoffee.mapper.EmpMapper">

    <resultMap id="EmpMap" type="Emp">
        <id column="id" property="id"></id>
        <id column="lastname" property="lastname"></id>
        <id column="email" property="email"></id>
        <id column="gender" property="gender"></id>
        <association property="dept" javaType="com.acoffee.pojo.Dept">
            <result property="pid" column="pid"></result>
            <result property="dname" column="dname"></result>
        </association>
    </resultMap>

    <select id="queryEmpList" resultMap="EmpMap">
        select emp.*,dept.* from emp_dept.employees emp,emp_dept.department dept where emp.did = dept.pid
    </select>

</mapper>

执行结果：

SpringBoot进行多表查询功能的实现