在项目中添加依赖,依赖已经包含了mybatis-plus-boot-starter<3.4.2>
依赖后无需再次引入mybatis-plus
```
<dependency>
    <groupId>com.github.yulichang</groupId>
    <artifactId>mybatis-plus-join</artifactId>
    <version>1.0.9</version>
</dependency>
```
或者clone代码到本地,执行mvn install,再引入以上依赖

配置插件,添加MPJInterceptor

@Configuration
public class MybatisPlusConfig {
    /**
     * 启用连表拦截器
     */
    @Bean
    public MybatisPlusInterceptor paginationInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        //分页插件
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor());
        //连表插件
        interceptor.addInnerInterceptor(new MPJInterceptor());
        //多租户,垃圾sql拦截插件......
        return interceptor;
    }
}

使用

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

MPJBaseMapper继承BaseMapper,在原有的方法基础上又添加了以下方法:
- selectJoinOne 连表查询一条记录对象
- selectJoinList 连表查询返回命中记录对象集合
- selectJoinPage 连表分页查询对象集合
- selectJoinMap 连表查询一条记录返回Map
- selectJoinMaps 连表查询返回命中记录Map集合
- selectJoinMapsPage 连表分页查询返回Map集合
MPJBaseService 继承了IService,同样添加以上方法
MPJBaseServiceImpl 继承了ServiceImpl,同样添加了以上方法

核心类 MPJQueryWrapper,MPJLambdaQueryWrapper和MPJJoinLambdaQueryWrapper

-	MPJQueryWrapper	MPJLambdaQueryWrapper	MPJJoinLambdaQueryWrapper
select(String)	支持	支持	不支持
select(lambda)	不支持	仅支持主表lambda	所有表lambda
join(String)	支持	支持	不支持
join(lambda)	不支持	不支持	支持
条件String	支持	不支持	不支持
条件lambda	不支持	仅支持主表lambda	所有表lambda

MPJQueryWrapper相当于mp的QueryWrapper
MPJLambdaQueryWrapper相当于mp的LambdaQueryWrapper

两者可以无缝切换
MPJQueryWrapper.lambda() ===> MPJLambdaQueryWrapper
MPJLambdaQueryWrapper.stringQuery() ===> MPJQueryWrapper

MPJJoinLambdaQueryWrapper用法

MPJJoinLambdaQueryWrapper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 MPJJoinLambdaQueryWrapper<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(true, UserDO::getId, 1)
                        .like(true, UserAddressDO::getTel, "1")
                        .gt(true, 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 MPJJoinLambdaQueryWrapper<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和MPJLambdaQueryWrapper

简单的3表查询

class test {
    @Resource
    private UserMapper userMapper;

    void testJoin() {
        List<UserDTO> list = userMapper.selectJoinList(UserDTO.class,
                new MPJLambdaQueryWrapper<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")
                        .gt(true, UserDO::getId, 1)
                        .eq(true, UserDO::getSex, "男")
                        .stringQuery()
                        .like(true, "addr.tel", "1")
                        .le(true, "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 (
    t.id > ?
    AND t.sex = ?
    AND 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)
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)
stringQuery() lambda查询转string查询
lambda() string查询转lambda查询
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 MPJLambdaQueryWrapper<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 MPJLambdaQueryWrapper<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")
                        .eq(true, UserDO::getId, 1)
                        .stringQuery()
                        .like(true, "addr.tel", "1")
                        .le(true, "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 (
    t.id = ?
    AND addr.tel LIKE ?
    AND a.province <= ?)
ORDER BY
    addr.id DESC