mybatis-plus-join

参考: https://gitee.com/best_handsome/mybatis-plus-join/wikis/%E5%AE%89%E8%A3%85

方法一

添加依赖

    com.github.yulichang
    mybatis-plus-join
    
    1.3.3

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方法二
clone代码到本地,执行mvn install,添加依赖

    com.github.yulichang
    mybatis-plus-join
    
    1.3.3

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使用

mapper继承MPJBaseMapper
@Mapper
public interface UserMapper extends MPJBaseMapper {

}
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(可选)service继承MPJBaseService
public interface UserService extends MPJBaseService {

}
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(可选)serviceImpl继承MPJBaseServiceImpl
@Service
public class UserServiceImpl extends MPJBaseServiceImpl implements UserService {

}
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selectCount

连表查询返回记录总数

MPJQueryWrapper
class MpJoinTest {
    @Resource
    private UserMapper userMapper;

    @Test
    void joinTest() {
        Integer count = userMapper.selectJoinCount(new MPJQueryWrapper()
                .leftJoin("user_address addr on addr.user_id = t.id")
                .eq("t.id", 2));
        System.out.println(count);
    }
}
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对应sql

SELECT COUNT( * ) FROM user t LEFT JOIN user_address addr on addr.user_id = t.id WHERE (t.id = ?)
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如果要执行count(id),可以调用select()系列的方法,但是只能有一个字段

class MpJoinTest {
    @Resource
    private UserMapper userMapper;

    @Test
    void joinTest() {
        Integer count = userMapper.selectJoinCount(new MPJQueryWrapper()
                .select("t.id")
                .leftJoin("user_address addr on addr.user_id = t.id")
                .eq("t.id", 2));
        System.out.println(count);
    }
}
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对应sql

SELECT COUNT( t.id ) FROM user t LEFT JOIN user_address addr on addr.user_id = t.id WHERE 
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(t.id = ?)
MPJLambdaWrapper
class MpJoinTest {
    @Resource
    private UserMapper userMapper;

    @Test
    void joinTest() {
        Integer count = userMapper.selectJoinCount(new MPJLambdaWrapper<>()
                .leftJoin(UserAddressDO.class, UserAddressDO::getUserId, UserDO::getId)
                .eq(UserDO::getId, 2));
        System.out.println(count);
    }
}
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对应sql

SELECT COUNT( * ) FROM user t LEFT JOIN user_address t1 ON (t1.user_id = t.id) WHERE (t.id = ?)
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如果要执行count(id),可以调用select()系列的方法,但是只能有一个字段

class MpJoinTest {
    @Resource
    private UserMapper userMapper;

    @Test
    void joinTest1() {
        Integer count = userMapper.selectJoinCount(new MPJLambdaWrapper<>()
                .select(UserDO::getId)
                .leftJoin(UserAddressDO.class, UserAddressDO::getUserId, UserDO::getId)
                .eq(UserDO::getId, 2));
        System.out.println(count);
    }
}
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对应sql

SELECT COUNT( t.id ) FROM user t LEFT JOIN user_address t1 ON (t1.user_id = t.id) WHERE (t.id = ?)
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MPJQueryWrapper

class MpJoinTest {
    @Resource
    private UserMapper userMapper;

    @Test
    void joinTest() {
        UserDTO dto = userMapper.selectJoinOne(UserDTO.class, new MPJQueryWrapper()
                .selectAll(UserDO.class)
                .select("addr.address")
                .leftJoin("user_address addr on addr.user_id = t.id")
                .eq("t.id", 2));
    }
}
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对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    addr.address,
FROM 
    user t 
    LEFT JOIN user_address addr on addr.user_id = t.id 
WHERE (
    t.id = ?)
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MPJLambdaWrapper

class MpJoinTest {
    @Resource
    private UserMapper userMapper;

    @Test
    void joinTest() {
        UserDTO dto = userMapper.selectJoinOne(UserDTO.class,
                new MPJLambdaWrapper<>()
                        .selectAll(UserDO.class)
                        .select(UserAddressDO::getAddress)
                        .leftJoin(UserAddressDO.class, UserAddressDO::getUserId,UserDO::getId)
                        .eq(UserDO::getId, 2));
    }
}
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对应sql

SELECT 
    t.id,
    t.name,
    t.sex,
    t.head_img,
    t1.address 
FROM 
    user t 
    LEFT JOIN user_address t1 ON t1.user_id = t.id 
WHERE (
    t.id = ?)
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注意:如果执行sql返回多条记录会如下报错,请自行处理

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.TooManyResultsException: Expected one result (or null) to be returned by selectOne(), but found: 2


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具体参考官网: https://gitee.com/best_handsome/mybatis-plus-join/wikis/API%E6%96%B9%E6%B3%95/SelectJoinOne
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