MyBatis Plus 连表查询

1.引入新的依赖

maven：

<dependency>
<groupId>com.github.yulichang</groupId>
<artifactId>mybatis-plus-join-boot-starter</artifactId>
<version>1.4.2.2</version>
</dependency>

gradle：

implementation 'com.github.yulichang:mybatis-plus-join-boot-starter:1.4.2.2'

*mybatis plus version >= 3.4.0

2.连表查询：

MPJLambdaWrapper

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")

• 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

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…

3.一对多查询

MPJLambdaWrapper<UserDO> wrapper = new MPJLambdaWrapper<UserDO>()
.selectAll(UserDO.class)
//一对多查询
.selectCollection(AddressDO.class, UesrDTO::getAddressList)
.leftJoin(AddressDO.class, AddressDO::getUserId, UserDO::getId);
List<UserDTO> dtoList = userMapper.selectJoinList(UserDTO.class, wrapper);

<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>

4.String形式用法

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);