MybatisPlus—QueryWrapper、QueryChainWrapper、LambdaQueryWrapper以及LambdaQueryChainWrapper

阅读完需：约 2 分钟

前言：以前都是使用QueryWrapper的情况多一点，其余三种也没怎么涉及。

首先准备一个对应的实体类：

@Data
@TableName("house")
public class HouseEntity {
    @TableId(type = IdType.AUTO)
    private Integer houseId;
    private String houseOwner;
    private String houseArea;
}

分别测试使用代码测试，都以查询所有的数据为目的。都注入

@Autowired
private HouseMapper houseMapper;

QueryWrapper

@Test
public void test1() {
List<HouseEntity> list=houseMapper.selectList(new QueryWrapper<HouseEntity>()
                .orderByDesc("house_area"));
        System.out.println("QueryWrapper:"+list);
    }

执行结果：QueryWrapper:[HouseEntity(houseId=2, houseOwner=LS, houseArea=72), HouseEntity(houseId=1, houseOwner=ZS, houseArea=70)]

最基础的使用方式是这样

// 查询条件构造器
QueryWrapper<HouseEntity> wrapper = new QueryWrapper<>();
wrapper.eq("houseOwner",house);
// 查询操作
List<HouseEntity> house = houseMapper.selectList(wrapper);

然后我们可以引入lambda，避免我们在代码中写类似的于banner_id的硬编码

QueryWrapper<HouseEntity> wrapper = new QueryWrapper<>();
wrapper.lambda().eq(HouseEntity::houseOwner, house);
List<HouseEntity> house = houseMapper.selectList(wrapper);

LambdaQueryWrapper

为了简化lambda的使用，我们可以改写成LambdaQueryWrapper构造器，语法如下：

@Test
public void test3() {
List<HouseEntity> list=houseMapper.selectList(new LambdaQueryWrapper<HouseEntity>()
                .orderByDesc(HouseEntity::getHouseArea));
       System.out.println("LambdaQueryWrapper:"+list);
    }

执行结果：LambdaQueryWrapper:[HouseEntity(houseId=2, houseOwner=LS, houseArea=72), HouseEntity(houseId=1, houseOwner=ZS, houseArea=70)]

或者

LambdaQueryWrapper<HouseEntity> wrapper = new QueryWrapper<HouseEntity>().lambda();
wrapper.eq(HouseEntity:: getHouseArea,house);
List<HouseEntity> house = houseMapper.selectList(wrapper);

我们可以再次将QueryWrapper<BannerItem>.lambda()简化，变成这个样子

LambdaQueryWrapper<HouseEntity> wrapper = new LambdaQueryWrapper<>();
wrapper.eq(BannerItem::getBannerId, id);
List<HouseEntity> house = houseMapper.selectList(wrapper);

QueryChainWrapper

MyBatis-Plus还提供了一种链式查询的方式，和上面的代码效果一样。 但是这种写法偏向于炫技，可读性没有上面的代码强。

@Test
public void test() {
List<HouseEntity> list2=new QueryChainWrapper<>(houseMapper)
                .orderByDesc("house_area").list();
        System.out.println("QueryChainWrapper:"+list);
    }

执行结果：QueryChainWrapper:[HouseEntity(houseId=2, houseOwner=LS, houseArea=72), HouseEntity(houseId=1, houseOwner=ZS, houseArea=70)]

LambdaQueryChainWrapper

@Test
public void test3() {
List<HouseEntity> list = new LambdaQueryChainWrapper<>(houseMapper)
                .orderByDesc(HouseEntity::getHouseArea).list();
        System.out.println("LambdaQueryChainWrapper:"+list);
  }
执行结果：LambdaQueryChainWrapper:[HouseEntity(houseId=2, houseOwner=LS, houseArea=72), HouseEntity(houseId=1, houseOwner=ZS, houseArea=70)]

总结：

1. QueryWrapper、QueryChainWrapper只能都只能指定需要的数据库列名
2. LambdaQueryWrapper、LambdaQueryChainWrapper可以通过获取Lambda数据库列名
3. QueryWrapper 、LambdaQueryWrapper不能使用链式查询的方式，必须借助BaseMapper来执行
4. QueryChainWrapper、LambdaQueryChainWrapper可以使用链式查询的方式，如list(),one()