MyBatis-Plus 查询返回实体对象还是map

  /**
   * 1. 名字包含雨并且年龄小于40
   * sql：name like '%雨%' and age < 40
   * <p>
   * 应用场景：
   * 当表字段非常多，但是你只需要查询少数几列,
   * 没必要返回的泛型为实体的list，如果返回的泛型为实体,绝大多字段都是null,这样做不优雅
   * 用返回泛型为map建议使用
   */
  @Test
  public void selectByWrapperMaps() {
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    queryWrapper.like("name", "雨").lt("age", 40);
    //建议使用
    List<Map<String, Object>> userList = userMapper.selectMaps(queryWrapper);
    //不建议使用
//    List<User> userList = userMapper.selectList(queryWrapper);
    userList.forEach(System.out::println);
  }
  /*
    sql形式:SELECT id,name,age,email,manager_id,create_time FROM user WHERE name LIKE ? AND age < ?
   */

  /**
   * 按照直属上级分组，查询每组的平均年龄。最大年龄、最小年龄。
   * 并且只取年龄总和小于500的组
   * select avg(age) avg_age,min(age) min_age,max(age) max_age from user
   * group by manager_id
   * having sum(age) < 500 ;
   */

  @Test
  public void selectByWrapperMaps2() {
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    queryWrapper.select("avg(age) avg_age", "min(age) min_age", "max(age) max_age")
        .groupBy("manager_id")
        .having("sum(age) < {0}", 500);

    List<Map<String, Object>> userList = userMapper.selectMaps(queryWrapper);
    userList.forEach(System.out::println);
//  sql形式：SELECT avg(age) avg_age,min(age) min_age,max(age) max_age FROM user
//  GROUP BY manager_id HAVING sum(age) < ?
}