一、mybatis-plus条件构造器

1、条件构造器关系图

1.橙色框为抽象类abstract
2.蓝色框为正常class类，可new对象
3.黄色箭头指向为父子类关系，箭头指向为父类

注意Service接口 都继承了 IService 的接口
Service

public interface SchoolStudentService  extends IService<SchoolStudent> {

}

ServiceImpl

@Service
public class SchoolStudentServiceImpl  extends ServiceImpl<SchoolStudentMapper, SchoolStudent> implements SchoolStudentService {

    @Resource
    private SchoolStudentMapper schoolStudentMapper;

}

Mapper

public interface SchoolStudentMapper extends BaseMapper<SchoolStudent> {

}

1、QueryWrapper

继承了AbstractWrapper ，自身属性内的entity实体类可作为where条件，可以通过 new QueryWrapper().lambda() 方法获取到 LambdaQueryWrapper

select方法

    public SchoolStudent querySchoolStudent3(Integer id) {
        //第一种只查"name"和"sex"两个字段
        QueryWrapper<SchoolStudent> schoolStudentQueryWrapper1 = new QueryWrapper<>();
        schoolStudentQueryWrapper1.select("name","sex");
        List<SchoolStudent> schoolStudentList1 = schoolStudentMapper.selectList(schoolStudentQueryWrapper1);
    }

select方法

        //第二种只排除"name"字段
        QueryWrapper<SchoolStudent> schoolStudentQueryWrapper2 = new QueryWrapper<>();
        schoolStudentQueryWrapper2.select(SchoolStudent.class, s -> {
            if ("name".equals(s.getColumn())) {
                return false;
            }
            return true;
        });
        List<SchoolStudent> schoolStudentList2 = schoolStudentMapper.selectList(schoolStudentQueryWrapper2);

2、UpdateWrapper

继承了AbstractWrapper ，自身属性内的entity实体类可作为where条件，可以通过 new UpdateWrapper().lambda() 方法获取到 LambdaQueryWrapper

set方法和setSql方法

        //set方法
        UpdateWrapper<SchoolStudent> studentUpdateWrapper1 = new UpdateWrapper<>();
        studentUpdateWrapper1.eq("id","1");
        studentUpdateWrapper1.set("age",id);
        // 设置sql语句部分
        studentUpdateWrapper1.setSql("sex = '男"+id+"'");
        //这里的this是serviceImpl
        this.update(studentUpdateWrapper1);

3、LambdaQueryWrapper

        LambdaQueryWrapper<SchoolStudent> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.eq(SchoolStudent::getId, id);
        List<SchoolStudent> schoolStudentList = this.list(lambdaQueryWrapper);

4、LambdaUpdateWrapper

        LambdaUpdateWrapper<SchoolStudent> updateWrapper = new LambdaUpdateWrapper<>();
        updateWrapper.eq(SchoolStudent::getId, id)
                .set(SchoolStudent::getSex, "男")
                .setSql("age = '" + id + "'");
        this.update(updateWrapper);

5、LambdaQueryChainWrapper

        List<SchoolStudent> list = new LambdaQueryChainWrapper<>(schoolStudentMapper).list();

二、其他