MyBatis-Plus实现连表查询的方法实例
作者：qq_31116181
这篇文章主要给大家介绍了关于MyBatis-Plus实现连表查询的相关资料,文中通过实例代码介绍的非常详细,对大家的学习或者工作具有一定的参考学习价值,需要的朋友可以参考下
mybatis-plus-join
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
总结
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