MyBatis-Plus实现连表查询

软件发布|下载排行|最新软件

当前位置:首页IT学院IT技术

MyBatis-Plus实现连表查询

qq_31116181   2022-05-23 我要评论

mybatis-plus-join

使用方法

安装

在项目中添加依赖

<dependency>
    <groupId>com.github.yulichang</groupId>
    <artifactId>mybatis-plus-join</artifactId>
    <version>1.2.2</version>
</dependency>

或者clone代码到本地,执行mvn install,再引入以上依赖

注意: mybatis plus version >= 3.4.0

使用

  • mapper继承MPJBaseMapper (必选)
  • service继承MPJBaseService (可选)
  • serviceImpl继承MPJBaseServiceImpl (可选)

核心类 MPJLambdaWrapper和MPJQueryWrapper

MPJLambdaWrapper用法

MPJLambdaWrapper示例

简单的3表查询

class test {
    @Resource
    private UserMapper userMapper;

    void testJoin() {
        List<UserDTO> list = userMapper.selectJoinList(UserDTO.class,
                new MPJLambdaWrapper<UserDO>()
                        .selectAll(UserDO.class)
                        .select(UserAddressDO::getTel)
                        .selectAs(UserAddressDO::getAddress, UserDTO::getUserAddress)
                        .select(AreaDO::getProvince, AreaDO::getCity)
                        .leftJoin(UserAddressDO.class, UserAddressDO::getUserId, UserDO::getId)
                        .leftJoin(AreaDO.class, AreaDO::getId, UserAddressDO::getAreaId)
                        .eq(UserDO::getId, 1)
                        .like(UserAddressDO::getTel, "1")
                        .gt(UserDO::getId, 5));
    }
}

对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    t1.tel,
    t1.address AS userAddress,
    t2.province,
    t2.city 
FROM 
    user t 
    LEFT JOIN user_address t1 ON t1.user_id = t.id 
    LEFT JOIN area t2 ON t2.id = t1.area_id 
WHERE (
    t.id = ? 
    AND t1.tel LIKE ? 
    AND t.id > ?)

说明:

  • UserDTO.class 查询结果返回类(resultType)
  • selectAll() 查询指定实体类的全部字段
  • select() 查询指定的字段,支持可变参数,同一个select只能查询相同表的字段
  • 故将UserAddressDO和AreaDO分开为两个select()
  • selectAs() 字段别名查询,用于数据库字段与业务实体类属性名不一致时使用
  • leftJoin() 参数说明
  • 第一个参数: 参与连表的实体类class
  • 第二个参数: 连表的ON字段,这个属性必须是第一个参数实体类的属性
  • 第三个参数: 参与连表的ON的另一个实体类属性
  • 默认主表别名是t,其他的表别名以先后调用的顺序使用t1,t2,t3…
  • 条件查询,可以查询主表以及参与连接的所有表的字段,全部调用mp原生的方法,正常使用没有sql注入风险

分页查询

class test {
    @Resource
    private UserMapper userMapper;

    void testJoin() {
        IPage<UserDTO> iPage = userMapper.selectJoinPage(new Page<>(2, 10), UserDTO.class,
                new MPJLambdaWrapper<UserDO>()
                        .selectAll(UserDO.class)
                        .select(UserAddressDO::getTel)
                        .selectAs(UserAddressDO::getAddress, UserDTO::getUserAddress)
                        .select(AreaDO::getProvince, AreaDO::getCity)
                        .leftJoin(UserAddressDO.class, UserAddressDO::getUserId, UserDO::getId)
                        .leftJoin(AreaDO.class, AreaDO::getId, UserAddressDO::getAreaId));
    }
}

对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    t1.tel,
    t1.address AS userAddress,
    t2.province,
    t2.city
FROM 
    user t
    LEFT JOIN user_address t1 ON t1.user_id = t.id
    LEFT JOIN area t2 ON t2.id = t1.area_id
LIMIT ?,?

MPJQueryWrapper

简单的3表查询

class test {
    @Resource
    private UserMapper userMapper;

    void testJoin() {
        List<UserDTO> list = userMapper.selectJoinList(UserDTO.class,
                new MPJQueryWrapper<UserDO>()
                        .selectAll(UserDO.class)
                        .select("addr.tel", "addr.address", "a.province")
                        .leftJoin("user_address addr on t.id = addr.user_id")
                        .rightJoin("area a on addr.area_id = a.id")
                        .like("addr.tel", "1")
                        .le("a.province", "1"));
    }
}

对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    addr.tel,
    addr.address,
    a.province
FROM 
    user t
    LEFT JOIN user_address addr on t.id = addr.user_id
    RIGHT JOIN area a on addr.area_id = a.id
WHERE (
    addr.tel LIKE ?
    AND a.province <= ?)

说明:

  • UserDTO.class 查询结果类(resultType)
  • selectAll(UserDO.class) 查询主表全部字段(主表实体类)默认主表别名 “t”
  • select() mp的select策略是覆盖,以最后一次为准,这里的策略是追加,可以一直select
  • 主表字段可以用lambda,会自动添加表别名,主表别名默认是 t ,非主表字段必须带别名查询
  • leftJoin() rightJoin() innerJoin() 传sql片段 格式 (表 + 别名 + 关联条件)
  • 条件查询,可以查询主表以及参与连接的所有表的字段,全部调用mp原生的方法,正常使用没有sql注入风险

分页查询

class test {
    @Resource
    private UserMapper userMapper;

    void testJoin() {
        IPage<UserDTO> page = userMapper.selectJoinPage(new Page<>(1, 10), UserDTO.class,
                new MPJQueryWrapper<UserDO>()
                        .selectAll(UserDO.class)
                        .select("addr.tel", "addr.address")
                        .select("a.province")
                        .leftJoin("user_address addr on t.id = addr.user_id")
                        .rightJoin("area a on addr.area_id = a.id"));
    }
}

对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    addr.tel,
    addr.address,
    a.province
FROM 
    user t
    LEFT JOIN user_address addr on t.id = addr.user_id
    RIGHT JOIN area a on addr.area_id = a.id 
LIMIT ?,?

还可以这么操作,但不建议

class test {
    @Resource
    private UserMapper userMapper;

    void testJoin() {
        List<UserDTO> list = userMapper.selectJoinList(UserDTO.class,
                new MPJQueryWrapper<UserDO>()
                        .selectAll(UserDO.class)
                        .select("addr.tel", "addr.address")
                        //行列转换
                        .select("CASE t.sex WHEN '男' THEN '1' ELSE '0' END AS sex")
                        //求和函数
                        .select("sum(a.province) AS province")
                        //自定义数据集
                        .leftJoin("(select * from user_address) addr on t.id = addr.user_id")
                        .rightJoin("area a on addr.area_id = a.id")
                        .like("addr.tel", "1")
                        .le("a.province", "1")
                        .orderByDesc("addr.id"));
    }
}

对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    addr.tel,
    addr.address,
    CASE t.sex WHEN '男' THEN '1' ELSE '0' END AS sex,
    sum(a.province) AS province
FROM 
    user t
    LEFT JOIN (select * from user_address) addr on t.id = addr.user_id
    RIGHT JOIN area a on addr.area_id = a.id
WHERE (
    addr.tel LIKE ?
    AND a.province <= ?)
ORDER BY
    addr.id DESC

总结

Copyright 2022 版权所有 软件发布 访问手机版

声明:所有软件和文章来自软件开发商或者作者 如有异议 请与本站联系 联系我们