MyBatis中高级多表查询(ResultMap、association、collection)详解

更新时间：2024年11月27日 17:12:05 作者：雷神乐乐

文章主要介绍了MyBatis中高级多表查询的四种方式：ResultMap、association、collection以及自连接查询,通过定义接口的抽象方法、编写mapper.xml和测试类,详细展示了如何根据复杂数据结构进行数据的装配和查询,感兴趣的朋友一起看看吧

一、ResultMap的使用

resultType：适用于装配数据时需要字段与属性一一对应(包含驼峰)
resultMap：适用于复杂数据结构。如果不指定里面的配置，其功能与resultType一致(自动对应)；如果指定了里面的配置，会根据手动指定的列与属性的对应关系装配数据

(一)定义接口的抽象方法

/**
* 查询每个员工的总工资
*/
List<Emp> empWithTotal();

(二)编写mapper.xml

<!--高级查询-->
<resultMap id="empBaseMap" type="com.javatest.entity.Emp">
    <id property="empno" column="empno"></id>
    <result property="ename" column="ename"></result>
    <result property="job" column="job"></result>
    <result property="mgr" column="mgr"></result>
    <result property="hiredate" column="hiredate"></result>
    <result property="sal" column="sal"></result>
    <result property="comm" column="comm"></result>
    <result property="deptno" column="deptno"></result>
</resultMap>
<resultMap id="empWithTotal" extends="empBaseMap" type="com.javatest.entity.Emp">
    <result property="sal" column="totalSal"></result>
</resultMap>
<select id="empWithTotal" resultMap="empWithTotal">
    select *, (sal + ifnull(comm, 0)) totalSal
    from emp;
</select>

(三)测试类

@Test
public void testEmpWithTotal() {
    SqlSession sqlSession = MyBatisHelper.getSqlSession();
    EmpDao mapper = sqlSession.getMapper(EmpDao.class);
    List<Emp> empList = mapper.empWithTotal();
    System.out.println(JSON.toJSONString(empList));
    MyBatisHelper.backAndSaveSqlSession(sqlSession);
}

[{"deptno":20,"empno":7369,"ename":"SMITH","hiredate":"1980-12-17","job":"CLERK","mgr":7902,"sal":800.0},
{"comm":300.0,"deptno":30,"empno":7499,"ename":"ALLEN","hiredate":"1981-02-20","job":"SALESMAN","mgr":7698,"sal":1900.0},
{"comm":500.0,"deptno":30,"empno":7521,"ename":"WARD","hiredate":"1981-02-22","job":"SALESMAN","mgr":7698,"sal":1750.0},
{"deptno":20,"empno":7566,"ename":"JONES","hiredate":"1981-04-02","job":"MANAGER","mgr":7839,"sal":2975.0},
{"comm":1400.0,"deptno":30,"empno":7654,"ename":"MARTIN","hiredate":"1981-09-28","job":"SALESMAN","mgr":7698,"sal":2650.0},
{"deptno":30,"empno":7698,"ename":"BLAKE","hiredate":"1981-05-01","job":"MANAGER","mgr":7839,"sal":2850.0},
{"deptno":10,"empno":7782,"ename":"CLARK","hiredate":"1981-06-09","job":"MANAGER","mgr":7839,"sal":2450.0},
{"deptno":20,"empno":7788,"ename":"SCOTT","hiredate":"1987-04-19","job":"ANALYST","mgr":7566,"sal":3000.0},
{"deptno":10,"empno":7839,"ename":"KING","hiredate":"1981-11-17","job":"PRESIDENT","sal":5000.0},
{"comm":0.0,"deptno":30,"empno":7844,"ename":"TURNER","hiredate":"1981-09-08","job":"SALESMAN","mgr":7698,"sal":1500.0},
{"deptno":20,"empno":7876,"ename":"ADAMS","hiredate":"1987-05-23","job":"CLERK","mgr":7788,"sal":1100.0},
{"deptno":30,"empno":7900,"ename":"JAMES","hiredate":"1981-12-03","job":"CLERK","mgr":7698,"sal":950.0},
{"deptno":20,"empno":7902,"ename":"FORD","hiredate":"1981-12-03","job":"ANALYST","mgr":7566,"sal":3000.0},
{"deptno":10,"empno":7934,"ename":"MILLER","hiredate":"1982-01-23","job":"CLERK","mgr":7782,"sal":1300.0}]

注意：这种方式改变了原有的sal属性的意思，开发中还是要使用DTO和VO来装配，这里简写。

二、多表一对一查询——association

(一)定义接口的抽象方法

/**
 * 查询每个员工对应的部门
 */
List<Emp> listEmpWithDept();

(二)编写mapper.xml

<!--高级查询-->
<resultMap id="empBaseMap" type="com.javatest.entity.Emp">
    <id property="empno" column="empno"></id>
    <result property="ename" column="ename"></result>
    <result property="job" column="job"></result>
    <result property="mgr" column="mgr"></result>
    <result property="hiredate" column="hiredate"></result>
    <result property="sal" column="sal"></result>
    <result property="comm" column="comm"></result>
    <result property="deptno" column="deptno"></result>
</resultMap>
<resultMap id="listEmpWithDept" extends="empBaseMap" type="com.javatest.entity.Emp">
    <!--配置关联的对象-->
    <association property="dept" javaType="com.javatest.entity.Dept">
        <id property="deptno" column="deptno"></id>
        <result property="dname" column="dname"></result>
        <result property="loc" column="loc"></result>
    </association>
</resultMap>
<select id="listEmpWithDept" resultMap="listEmpWithDept">
    select *
    from dept d
             left join emp e on d.deptno = e.deptno
</select>

(三)测试类

@Test
public void testListEmpWithDept() {
    SqlSession sqlSession = MyBatisHelper.getSqlSession();
    EmpDao mapper = sqlSession.getMapper(EmpDao.class);
    List<Emp> empList = mapper.listEmpWithDept();
    System.out.println(JSON.toJSONString(empList));
    MyBatisHelper.backAndSaveSqlSession(sqlSession);
}

注意：真实开发中，数据库对应的类属性，是和数据库字段一一对应的，如果需要改变，那么就要放在VO类中返回给前端，这里简写。

三、多表一对多查询——collection

(一)定义接口的抽象方法

public interface DeptDao {
    /**
     * 查询每个部门对应的员工
     */
    List<Dept> listDeptWithEmps();
}

(二)编写mapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.javatest.dao.DeptDao">
    <resultMap id="DeptBaseMap" type="com.javatest.entity.Dept">
        <result property="deptno" column="deptno"/>
        <result property="dname" column="dname"/>
        <result property="loc" column="loc"/>
    </resultMap>
    <!--一对多查询-->
    <resultMap id="listDeptWithEmps" extends="DeptBaseMap" type="com.javatest.entity.Dept">
        <!--配置关联的集合-->
        <collection property="emps" ofType="com.javatest.entity.Emp">
            <id property="empno" column="empno"></id>
            <result property="ename" column="ename"></result>
            <result property="job" column="job"></result>
            <result property="mgr" column="mgr"></result>
            <result property="hiredate" column="hiredate"></result>
            <result property="sal" column="sal"></result>
            <result property="comm" column="comm"></result>
            <result property="deptno" column="deptno"></result>
        </collection>
    </resultMap>
    <select id="listDeptWithEmps" resultMap="listDeptWithEmps">
        select *
        from dept d
                 left join emp e on d.deptno = e.deptno
    </select>
</mapper>

(三)测试类

@Test
public void testListDeptWithEmps(){
    SqlSession sqlSession = MyBatisHelper.getSqlSession();
    DeptDao mapper = sqlSession.getMapper(DeptDao.class);
    List<Dept> deptList = mapper.listDeptWithEmps();
    System.out.println(JSON.toJSONString(deptList));
    MyBatisHelper.backAndSaveSqlSession(sqlSession);
}

四、自连接查询——省市县区

(一)tb_area表

create table if not exists tb_area
(
    code  int          not null primary key,
    name  varchar(255) null,
    pcode int          null
);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (110000, '北京市', 0);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (110107, '石景山区', 110000);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (210000, '辽宁省', 0);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (210200, '大连市', 210000);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (210202, '中山区', 210200);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (210400, '抚顺市', 210000);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (210402, '新抚区', 210400);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (310000, '上海市', 0);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (310106, '静安区', 310000);
INSERT INTO mydb.tb_area (code, name, pcode) VALUES (310110, '杨浦区', 310000);

(二)TbArea实体类

@Data
@NoArgsConstructor
@AllArgsConstructor
public class TbArea implements Serializable {
    private static final long serialVersionUID = 949170561403426696L;
    private Integer code;
    private String name;
    private Integer pcode;
    private List<TbArea> subArea;
}

(三)TbAreaDao接口

public interface TbAreaDao {
    /**
     * 查找省对应的市
     */
    List<TbArea> getCities();
    /**
     * 查找省对应的市，市对应的县区
     */
    List<TbArea> getAreas();
}

(四)TbArea.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.javatest.dao.TbAreaDao">
    <!--基础映射对象-->
    <resultMap id="tbArea" type="com.javatest.entity.TbArea">
        <id property="code" column="code"></id>
        <result property="name" column="name"></result>
        <result property="pcode" column="pcode"></result>
    </resultMap>
    <!--配置关联的集合：省-市-->
    <resultMap id="getCities" extends="tbArea" type="com.javatest.entity.TbArea">
        <collection property="subArea" ofType="com.javatest.entity.TbArea">
            <id property="code" column="cityCode"></id>
            <result property="name" column="cityName"></result>
            <result property="pcode" column="citypCode"></result>
        </collection>
    </resultMap>
    <!--配置关联的集合：省-市-县/区-->
    <resultMap id="getAreas" extends="tbArea" type="com.javatest.entity.TbArea">
        <collection property="subArea" ofType="com.javatest.entity.TbArea">
            <id property="code" column="cityCode"></id>
            <result property="name" column="cityName"></result>
            <result property="pcode" column="citypCode"></result>
            <collection property="subArea" ofType="com.javatest.entity.TbArea">
                <id property="code" column="areaCode"></id>
                <result property="name" column="areaName"></result>
                <result property="pcode" column="areapCode"></result>
            </collection>
        </collection>
    </resultMap>
    <!--获取省-市信息-->
    <select id="getCities" resultMap="getCities">
        select prov.*,
               city.code  as cityCode,
               city.name  as cityName,
               city.pcode as cityPcode
        from tb_area prov
                 left join tb_area city on prov.code = city.pcode
        where prov.pcode = 0
    </select>
    <!--获取省-市-县区信息-->
    <select id="getAreas" resultMap="getAreas">
        select prov.*,
               city.code  as cityCode,
               city.name  as cityName,
               city.pcode as cityPcode,
               area.code  as areaCode,
               area.name  as areaName,
               area.pcode as areaPcode
        from tb_area prov
                 left join tb_area city on prov.code = city.pcode
                 left join tb_area area on city.code = area.pcode
        where prov.pcode = 0
    </select>
</mapper>

sql查询结果：

获取省-市数据：

获取省-市-县区数据：

(五)测试类TbAreaTest

public class TbAreaTest {
    @Test
    public void testGetCities(){
        SqlSession sqlSession = MyBatisHelper.getSqlSession();
        TbAreaDao mapper = sqlSession.getMapper(TbAreaDao.class);
        List<TbArea> cities = mapper.getCities();
        System.out.println(JSON.toJSONString(cities));
        MyBatisHelper.backAndSaveSqlSession(sqlSession);
    }
    @Test
    public void testGetAreas(){
        SqlSession sqlSession = MyBatisHelper.getSqlSession();
        TbAreaDao mapper = sqlSession.getMapper(TbAreaDao.class);
        List<TbArea> areas = mapper.getAreas();
        System.out.println(JSON.toJSONString(areas));
        MyBatisHelper.backAndSaveSqlSession(sqlSession);
    }
}

testGetCities：获取省-市数据
[{"code":110000,"name":"北京市","pcode":0,"subArea":[{"code":110107,"name":"石景山区","pcode":110000}]},
{"code":210000,"name":"辽宁省","pcode":0,"subArea":[{"code":210400,"name":"抚顺市","pcode":210000},{"code":210200,"name":"大连市","pcode":210000}]},
{"code":310000,"name":"上海市","pcode":0,"subArea":[{"code":310110,"name":"杨浦区","pcode":310000},{"code":310106,"name":"静安区","pcode":310000}]}]
testGetAreas：获取省-市-县区数据
[{"code":110000,"name":"北京市","pcode":0,"subArea":[{"code":110107,"name":"石景山区","pcode":110000,"subArea":[]}]},
{"code":210000,"name":"辽宁省","pcode":0,"subArea":[{"code":210400,"name":"抚顺市","pcode":210000,"subArea":[{"code":210402,"name":"新抚区","pcode":210400}]},
{"code":210200,"name":"大连市","pcode":210000,"subArea":[{"code":210202,"name":"中山区","pcode":210200}]}]},
{"code":310000,"name":"上海市","pcode":0,"subArea":[{"code":310110,"name":"杨浦区","pcode":310000,"subArea":[]},{"code":310106,"name":"静安区","pcode":310000,"subArea":[]}]}]

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