将行转置为MySQL列[重复]

Question

我的查询提供了所有城市每月加入的员工人数。 （在 MySQL 中）

SELECT SUBSTR(`JoiningDate`,4,3) as Month,
 Location,
 Count(*) as `EmployeeCount` 
 FROM `data` 
 WHERE Location IN ('NYC','SFO','LA')   
 GROUP BY Month,Location

输出是这种格式 -

| Month|Location| Count|
-----------------------
|  Jan | NYC    |   100|
|  Jan | SFO    |   500|
|  Jan | LA     |   200|
|  Feb | NYC    |   100|
|  Feb | SFO    |   400|
|  Feb | LA     |   500|

我如何能够以某种格式转换数据，它会为每个城市生成具有每月相关计数的列？像下面这样 -

| Month|NYC |SFO | LA |
-----------------------
|  Jan |100 |500 |200 |

|  Feb |100 |400 |500 |

|  Mar |300 |300 |650 |

Answer 1

您可以使用条件聚合：

select SUBSTR(`JoiningDate`, 4, 3) as Month,
    sum(location = 'NYC') as `NYC`,
    sum(location = 'SFO') as `SFO`,
    sum(location = 'LA' ) as `LA`
from `data`
where Location in ('NYC', 'SFO', 'LA')
group by SUBSTR(`JoiningDate`, 4, 3)

它使用了在 MySQL 中 true 评估为 1 并且 false 评估为 0 的事实，您可以对它们进行求和以找到所需的计数。