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mysql - MySQL Count and Convert Row to Colum Involve One Table Only

问题描述

I have a table name histories that record user activities which consists of user_id, branch_id and duration.

The table look like this

+++++++++++++++++++++++++++++++++++
id | user_id | branch_id | totHours
+++++++++++++++++++++++++++++++++++
|1 |   100   |     1     |    1   |
|2 |   199   |     1     |    1   |
|3 |   121   |     1     |    1   |
|4 |   140   |     1     |    1   |
|5 |   103   |     2     |    3   |
|6 |   107   |     2     |    1   |
|7 |   299   |     1     |    2   |
|8 |   209   |     2     |    2   |
|9 |   119   |     1     |    5   |

I would like to produce an output like this:

+++++++++++++++++++++++++++
Hours | Branch A | Branch B
+++++++++++++++++++++++++++
|1    |    4    |     1   |
|2    |    1    |     1   |
|3    |    0    |     1   |
|4    |    0    |     0   |
|5    |    1    |     0   |

I try make it using this query, but when i use group by on totHours column only, it return error because i need to include the branch_id in the group by.

Here is my query:

select totHours as Hours,
   coalesce(case when branch_id = 1 then count(totHours) else 0 end) as 'Branch A',
   coalesce(case when branch_id = 2 then count(totHours) else 0 end) as 'Branch B'
from histories
group by totHours, branch_id;

And if the totHours is not in the table (for example in this table 4), it will display 0 for both branch column.

Here is my db fiddle

Update: MySQL version 5.7.22

Thanks

标签： mysql

解决方案

如果您使用 MySQL 版本 8+（或任何版本支持 windows 功能），您可以使用递归公用表表达式为您生成小时值，然后使用它LEFT JOIN表histories。之后，您可以SUM()使用CASE表达式SELECT来生成预期的输出：

WITH RECURSIVE hours AS (
    SELECT 1 AS hr, MAX(totHours) AS maxth FROM histories UNION ALL
    SELECT hr+1, maxth FROM hours WHERE hr+1 <= maxth)
SELECT hours.hr, 
       SUM(CASE WHEN histories.branch_id=1 THEN 1 ELSE 0 END) AS Branch_A,
       SUM(CASE WHEN histories.branch_id=2 THEN 1 ELSE 0 END) AS Branch_B
FROM hours
 LEFT JOIN histories 
 ON hours.hr=histories.totHours
GROUP BY hours.hr;

如果您使用的版本不支持窗口函数，您可以创建一个子查询来表示小时数（包括缺少的小时数）。这是一种硬编码方法，您可能必须始终更新子查询以包含新的小时值（如果有）：

SELECT hours.hr, 
       SUM(CASE WHEN histories.branch_id=1 THEN 1 ELSE 0 END) AS Branch_A,
       SUM(CASE WHEN histories.branch_id=2 THEN 1 ELSE 0 END) AS Branch_B
FROM 
    (SELECT 1 hr UNION 
      SELECT 2 UNION 
      SELECT 3 UNION 
      SELECT 4 UNION 
      SELECT 5) AS hours
 LEFT JOIN histories 
 ON hours.hr=histories.totHours
GROUP BY hours.hr;

演示小提琴

编辑hours子查询以添加更多，例如，如果您想要直到 7，您只需添加：

(SELECT 1 hr UNION 
      SELECT 2 UNION 
      SELECT 3 UNION 
      SELECT 4 UNION 
      SELECT 5 UNION 
      SELECT 6 UNION 
SELECT 7) AS hours

到子查询。另一种方法是预先定义小时数并创建参考表。假设您估计小时为 100，那么最好创建一个存储 1-100 的表作为参考LEFT JOIN

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