MyBatis-Plus如何实现连表查询
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使用方法
安装
在项目中添加依赖
com.github.yulichang mybatis-plus-join 1.2.2
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 list = userMapper.selectJoinList(UserDTO.class, new MPJLambdaWrapper() .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 iPage = userMapper.selectJoinPage(new Page<>(2, 10), UserDTO.class, new MPJLambdaWrapper() .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.cityFROM user t LEFT JOIN user_address t1 ON t1.user_id = t.id LEFT JOIN area t2 ON t2.id = t1.area_idLIMIT ?,?
MPJQueryWrapper
简单的3表查询
class test { @Resource private UserMapper userMapper; void testJoin() { List list = userMapper.selectJoinList(UserDTO.class, new MPJQueryWrapper() .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.provinceFROM user t LEFT JOIN user_address addr on t.id = addr.user_id RIGHT JOIN area a on addr.area_id = a.idWHERE ( 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 page = userMapper.selectJoinPage(new Page<>(1, 10), UserDTO.class, new MPJQueryWrapper() .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.provinceFROM 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 list = userMapper.selectJoinList(UserDTO.class, new MPJQueryWrapper() .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 provinceFROM user t LEFT JOIN (select * from user_address) addr on t.id = addr.user_id RIGHT JOIN area a on addr.area_id = a.idWHERE ( addr.tel LIKE ? AND a.province <= ?)ORDER BY addr.id DESC
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