你还在手写 join 联表查询？MyBatis-Plus 这样写太香了！
Posted Java技术栈
tags:
篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了你还在手写 join 联表查询？MyBatis-Plus 这样写太香了！相关的知识，希望对你有一定的参考价值。
点击关注公众号，Java干货及时送达
推荐阅读：
使用方法
安装
Maven
<dependency>
<groupId>com.github.yulichang</groupId>
<artifactId>mybatis-plus-join</artifactId>
<version>1.2.4</version>
</dependency>Gradle
implementation 'com.github.yulichang:mybatis-plus-join:1.2.4'或者clone代码到本地执行 mvn install, 再引入以上依赖
推荐一个开源免费的 Spring Boot 最全教程：
https://github.com/javastacks/spring-boot-best-practice
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
简单的三表查询
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:
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()
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
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 <= ?)另外，如果你近期准备面试跳槽，建议在Java面试库小程序在线刷题，涵盖 2000+ 道 Java 面试题，几乎覆盖了所有主流技术面试题。
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作者：阳宗德
链接：https://juejin.cn/post/7110405284811522085
End
Spring Cloud 微服务最新课程！
以上是关于你还在手写 join 联表查询？MyBatis-Plus 这样写太香了！的主要内容，如果未能解决你的问题，请参考以下文章
DQL查询数据----联表查询（left/right/inner join 区别）
DQL查询数据----联表查询（left/right/inner join 区别）