- 对 mybatis-plus 多表查询的扩展
- 演示工程
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- Maven
<dependency> <groupId>com.github.yulichang</groupId> <artifactId>mybatis-plus-join-boot-starter</artifactId> <version>1.4.2.2</version> </dependency>
- Gradle
或者clone代码到本地执行 mvn install, 再引入以上依赖
implementation 'com.github.yulichang:mybatis-plus-join-boot-starter:1.4.2.2'
注意: mybatis plus version >= 3.4.0
- mapper继承MPJBaseMapper (必选)
- service继承MPJBaseService (可选)
- serviceImpl继承MPJBaseServiceImpl (可选)
class test {
@Resource
private UserMapper userMapper;
void testJoin() {
//和Mybatis plus一致，MPJLambdaWrapper的泛型必须是主表的泛型，并且要用主表的Mapper来调用
MPJLambdaWrapper<UserDO> wrapper = new MPJLambdaWrapper<UserDO>()
.selectAll(UserDO.class)//查询user表全部字段
.select(UserAddressDO::getTel)//查询user_address tel 字段
.selectAs(UserAddressDO::getAddress, UserDTO::getUserAddress)//别名 t.address AS userAddress
.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);
//连表查询 返回自定义ResultType
List<UserDTO> list = userMapper.selectJoinList(UserDTO.class, wrapper);
//分页查询 （需要启用 mybatis plus 分页插件）
Page<UserDTO> listPage = userMapper.selectJoinPage(new Page<>(2, 10), UserDTO.class, wrapper);
}
}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
- 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
class test {
@Resource
private UserMapper userMapper;
@Test
void testResultMap() {
MPJLambdaWrapper<UserDO> wrapper = new MPJLambdaWrapper<UserDO>()
.selectAll(UserDO.class)
//对多查询
.selectCollection(AddressDO.class, UesrDTO::getAddressList)
//对一查询
.selectAssociation(AddressDO.class, UesrDTO::getAddress)
.leftJoin(AddressDO.class, AddressDO::getUserId, UserDO::getId);
List<UserDTO> dtoList = userMapper.selectJoinList(UserDTO.class, wrapper);
//关于对多分页查询
//由于嵌套结果方式会导致结果集被折叠，因此分页查询的结果在折叠后总数会减少，所以无法保证分页结果数量正确。
}
}等效于ResultMap
<resultMap id="xxxxxxxx" type="com.github.yulichang.join.dto.UserDTO">
<result property="id" column="id"/>
<result property="name" column="name"/>
<!--其他属性省略-->
<collection property="addressList" javaType="java.util.List"
ofType="com.github.yulichang.join.entity.UserAddressDO">
<id property="id" column="mpj_id"/>
<result property="address" column="address"/>
<result property="userId" column="user_id"/>
<!--其他属性省略-->
</collection>
</resultMap>MPJLambdaWrapper其他功能
class test {
@Resource
private UserMapper userMapper;
void testJoin() {
MPJQueryWrapper wrapper = 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");
//列表查询
List<UserDTO> list = userMapper.selectJoinList(UserDTO.class, wrapper);
//分页查询 （需要启用 mybatis plus 分页插件）
Page<UserDTO> listPage = userMapper.selectJoinPage(new Page<>(1, 10), UserDTO.class, wrapper);
}
}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
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