如何将两个查询与case语句结合起来答案

【问题标题】：How to combine two queries with case statement如何将两个查询与case语句结合起来
【发布时间】：2019-12-31 08:54:23
【问题描述】：

如何将两个查询与case语句结合起来

查询1

SELECT   DATE(created_at),COUNT(DISTINCT user_id) AS count1 
FROM     user
WHERE    by_user = '3'  
AND      DATE(created_at) BETWEEN CURRENT_DATE-2 AND CURRENT_DATE
GROUP BY DATE(created_at)

查询2

SELECT   DATE(created_at),COUNT(DISTINCT user_id) AS count2 
FROM     USER  
WHERE    by_user <> '3'  
AND      DATE(created_at) BETWEEN CURRENT_DATE-2 AND CURRENT_DATE
GROUP BY DATE(created_at)

预期输出：

+---------------------+--------+--------+
| date(created_at)    | count1 | count2 |
+---------------------+--------+--------+
| 30/12/2019,12:00 AM | 4000   | 5000   |
| 29/12/2019,12:00 AM | 2434   | 5432   |
+---------------------+--------+--------+

【问题讨论】：

标签： mysql sql date count pivot

【解决方案1】：

使用条件聚合：

select date(created_at), 
  count(distinct case when by_user = '3' then user_id end) as count1, 
  count(distinct case when by_user <> '3' then user_id end) as count2 
from user  
where date(created_at) between current_date-2 and current_date
group by date(created_at)

条件where by_user = '3' 或where by_user <> '3' 使用CASE 表达式进行检查，因此它们已从WHERE 子句中删除。

【讨论】：

【解决方案2】：

你可以做条件聚合：

select 
    date(created_at) day_at,
    count(distinct case when by_user = 3 then user_id end) as count1, 
    count(distinct case when by_user <> 3 then user_id end) as count2 
from user  
where 
    created_at >= current_date - interval 2 day 
    and created_at < current_date + interval 1 day
group by day_at
order by day_at

注意事项：

如果列by_user 是数字数据类型，则要进行数字比较，而不是字符串比较
我建议在过滤时不要将日期函数应用于created_at，而是使用半开间隔（这是一种索引友好的策略）。我相应地更改了您的 where 子句，还修改了您的日期算术以使用标准的 interval 语法。

【讨论】：