MyBatis-Plus 如何实现连表查询[通俗易懂]

全栈程序员站长

发布于 2022-10-05 10:08:14

1.8K0

发布于 2022-10-05 10:08:14

文章被收录于专栏：全栈程序员必看

大家好，又见面了，我是你们的朋友全栈君。

mybatis-plus-join

gitee github

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

本文参与腾讯云自媒体同步曝光计划，分享自作者个人站点/博客。

原始发表：2022年9月14日，如有侵权请联系 cloudcommunity@tencent.com 删除

sql