这篇文章跟大家分析一下“MyBatis-Plus如何实现连表查询”。内容详细易懂，对“MyBatis-Plus如何实现连表查询”感兴趣的朋友可以跟着小编的思路慢慢深入来阅读一下，希望阅读后能够对大家有所帮助。下面跟着小编一起深入学习“MyBatis-Plus如何实现连表查询”的知识吧。
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.selectJoinListUserDTO.class,
new MPJLambdaWrapper<UserDO>)
.selectAllUserDO.class)
.selectUserAddressDO::getTel)
.selectAsUserAddressDO::getAddress, UserDTO::getUserAddress)
.selectAreaDO::getProvince, AreaDO::getCity)
.leftJoinUserAddressDO.class, UserAddressDO::getUserId, UserDO::getId)
.leftJoinAreaDO.class, AreaDO::getId, UserAddressDO::getAreaId)
.eqUserDO::getId, 1)
.likeUserAddressDO::getTel, "1")
.gtUserDO::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.selectJoinPagenew Page<>2, 10), UserDTO.class,
new MPJLambdaWrapper<UserDO>)
.selectAllUserDO.class)
.selectUserAddressDO::getTel)
.selectAsUserAddressDO::getAddress, UserDTO::getUserAddress)
.selectAreaDO::getProvince, AreaDO::getCity)
.leftJoinUserAddressDO.class, UserAddressDO::getUserId, UserDO::getId)
.leftJoinAreaDO.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.selectJoinListUserDTO.class,
new MPJQueryWrapper<UserDO>)
.selectAllUserDO.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)
-
selectAllUserDO.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.selectJoinPagenew Page<>1, 10), UserDTO.class,
new MPJQueryWrapper<UserDO>)
.selectAllUserDO.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.selectJoinListUserDTO.class,
new MPJQueryWrapper<UserDO>)
.selectAllUserDO.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"suma.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, suma.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