【MySQL】union (all) 后 order by 子查询排序不生效问题解决方案
力扣题
1、题目地址
2308. 按性别排列表格
2、模拟表
表：Genders
| Column Name | Type |
|---|---|
| user_id | int |
| gender | varchar |
- user_id 是该表的主键(具有唯一值的列)。
- gender 的值是 ‘female’，‘male’，‘other’ 之一。
- 该表中的每一行都包含用户的 ID 及其性别。
- 表格中 ‘female’，‘male’，‘other’ 数量相等。
3、要求
编写一个解决方案以重新排列 Genders 表，使行按顺序在 ‘female’，‘other’ 和 ‘male’ 之间交替。同时每种性别按照 user_id 升序进行排序。
按 上述顺序 返回结果表。
返回结果格式如以下示例所示。
示例 1：
输入：
Genders 表：
| user_id | gender |
|---|---|
| 15 | other |
| 12 | female |
| 1 | other |
| 2 | female |
| 21 | other |
| 5 | male |
| 20 | male |
| 14 | other |
| 11 | male |
| 4 | male |
| 19 | male |
| 18 | other |
| 6 | other |
| 9 | female |
| 3 | female |
| 8 | female |
| 10 | male |
| 7 | other |
| 16 | male |
| 13 | female |
| 17 | female |
输出：
| user_id | gender |
|---|---|
| 2 | female |
| 1 | other |
| 4 | male |
| 3 | female |
| 6 | other |
| 5 | male |
| 8 | female |
| 7 | other |
| 10 | male |
| 9 | female |
| 14 | other |
| 11 | male |
| 12 | female |
| 15 | other |
| 16 | male |
| 13 | female |
| 18 | other |
| 19 | male |
| 17 | female |
| 21 | other |
| 20 | male |
解释：
女性：ID 2、3、8、9、12、13、17。
其他性别：ID 1、6、7、14、15、18、21。
男性：ID 4、5、10、11、16、19、20。
我们在 ‘female’，‘other’,‘male’ 之间交替排列表。
注意，每种性别都是按 user_id 升序排序的。
4、代码编写
错误写法（union (all) 后 order by 子查询排序不生效）
SELECT user_id, gender
FROM (
SELECT row_number() over (order by user_id) AS id, user_id, gender
FROM Genders
WHERE gender = 'female'
UNION ALL
SELECT row_number() over (order by user_id) AS id, user_id, gender
FROM Genders
WHERE gender = 'other'
UNION ALL
SELECT row_number() over (order by user_id) AS id, user_id, gender
FROM Genders
WHERE gender = 'male'
) AS one
ORDER BY id
| user_id | gender |
| ------- | ------ |
| 4 | male |
| 1 | other |
| 2 | female |
| 3 | female |
| 5 | male |
| 6 | other |
| 8 | female |
| 10 | male |
| 7 | other |
| 14 | other |
| 11 | male |
| 9 | female |
| 15 | other |
| 12 | female |
| 16 | male |
| 18 | other |
| 13 | female |
| 19 | male |
| 21 | other |
| 17 | female |
| 20 | male |
正确写法（在 union (all) 前后都加一个字段来保证顺序，后面在 order by 加上就行）
SELECT user_id, gender
FROM (
SELECT row_number() over (order by user_id) AS id, user_id, gender, 0 AS sort
FROM Genders
WHERE gender = 'female'
UNION ALL
SELECT row_number() over (order by user_id) AS id, user_id, gender, 1 AS sort
FROM Genders
WHERE gender = 'other'
UNION ALL
SELECT row_number() over (order by user_id) AS id, user_id, gender, 2 AS sort
FROM Genders
WHERE gender = 'male'
) AS one
ORDER BY id, sort
| user_id | gender |
| ------- | ------ |
| 2 | female |
| 1 | other |
| 4 | male |
| 3 | female |
| 6 | other |
| 5 | male |
| 8 | female |
| 7 | other |
| 10 | male |
| 9 | female |
| 14 | other |
| 11 | male |
| 12 | female |
| 15 | other |
| 16 | male |
| 13 | female |
| 18 | other |
| 19 | male |
| 17 | female |
| 21 | other |
| 20 | male |
网友写法（用的很巧妙）
SELECT *
FROM Genders
ORDER BY row_number() over(partition by gender order by user_id),
case when gender = 'female' then 1
when gender = 'male' then 3
else 2 end
思考得出的新写法
直接使用 order by gender 是不行的，female-male-other 的顺序不符合要求 female-other-male，刚好可以使用长度倒序去做
SELECT *
FROM genders
ORDER BY row_number() over(partition by gender order by user_id),
length(gender) desc
参考
mysql union (all) 后 order by 的排序失效问题解决