5.统计每个班级男女生人数最多的3条记录（去null值）

如果你想统计每个班级男女各自人数最多的情况，并保留三条记录（去除了null值），这通常涉及到两个步骤：首先，分别统计男生和女生在每个班级的数量；然后，取每个班级人数最多的前三名。这在SQL中可以表示为：

WITH male_counts AS (
  SELECT class_id, gender, COUNT(*) as count
  FROM users_info
  WHERE gender = '男' AND age IS NOT NULL
  GROUP BY class_id, gender
),
female_counts AS (
  SELECT class_id, gender, COUNT(*) as count
  FROM users_info
  WHERE gender = '女' AND age IS NOT NULL
  GROUP BY class_id, gender
)
SELECT m.class_id, '男' as gender, m.count
FROM male_counts m
JOIN (SELECT class_id, MAX(count) as max_male_count
      FROM male_counts
      GROUP BY class_id) mm ON m.class_id = mm.class_id AND m.count = mm.max_male_count
LIMIT 3

UNION ALL

SELECT f.class_id, '女' as gender, f.count
FROM female_counts f
JOIN (SELECT class_id, MAX(count) as max_female_count
      FROM female_counts
      GROUP BY class_id) mf ON f.class_id = mf.class_id AND f.count = mf.max_female_count
LIMIT 3;

这个查询创建了两个窗口函数子查询（male_counts 和 female_counts），用于计算每个班级的男生和女生人数，然后通过`JOIN`和`MAX()`函数找到每个班级人数最多的记录，并使用`UNION ALL`合并结果，最后限制返回的结果集最多为每种性别各三条记录。