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MyBatis-Plus实现连表查询的方法实例

这篇文章主要给大家介绍了关于MyBatis-Plus实现连表查询的相关资料,文中通过实例代码介绍的非常详细,对大家的学习或者工作具有一定的参考学习价值,需要的朋友可以参考下

目录

  • 使用方法

    • 安装

    • 使用

  • 核心类 MPJLambdaWrapper和MPJQueryWrapper

    • MPJLambdaWrapper用法

    • MPJQueryWrapper

  • 总结

    mybatis-plus-join

    • gitee

    • github

    使用方法

    安装

    在项目中添加依赖

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    <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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

    到此这篇关于MyBatis-Plus实现连表查询的文章就介绍到这了

    原文链接：https://blog.csdn.net/qq_31116181/article/details/113741489


    文章分类
    代码人生
    文章标签
    mybatis
    plus
    方法
    实例
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