Question

我有一个名为出勤的表，有2个属性（id，备注）。我想从出勤表中显示缺席或迟到的每个身份证。

Attendance Table
|ID         | Remarks       |
=============================
|1          | Absent        |
|1          | Late          |
|2          | Absent        |
|2          | Absent        |
|3          | Late          |

Sample Output
|ID         | Absent   | Late    |
==================================
|1          | 1        | 1       |
|2          | 2        |         |
|3          |          | 1       |

目前，我只能使用以下代码输出2列（ID和Absent）或（ID和Late）：

SELECT id, count(remarks) AS Absent 
FROM attendance 
WHERE remarks = 'Absent' 
GROUP BY id;

我不能同时显示缺席和晚期列..请帮忙。感谢。

Answer 1

这基本上是PIVOT。如果您无法访问PIVOT函数，则可以使用聚合函数和CASE语句复制它：

select id,
  sum(case when remarks = 'Absent' then 1 else 0 end) Absent,
  sum(case when remarks = 'Late' then 1 else 0 end) Late
from attendance
group by id

请参阅SQL Fiddle with Demo

或者您可以使用COUNT()：

select id,
  count(case when remarks = 'Absent' then 1 else null end) Absent,
  count(case when remarks = 'Late' then 1 else null end) Late
from attendance
group by id;

请参阅SQL Fiddle with Demo

Answer 2

使用SUM(CASE)构造分隔Absent和Late。对于每一个，如果值匹配，则CASE返回1或0，然后通过汇总SUM()将这些值相加，得出总数。这里的概念被称为pivot table。

SELECT
  id,
  SUM(CASE WHEN Remarks = 'Absent' THEN 1 ELSE 0 END) AS Absent,
  SUM(CASE WHEN Remarks = 'Late' THEN 1 ELSE 0 END) AS Late
FROM
  attendance
GROUP BY id

Answer 3

尝试：

 SELECT id, 
     Sum (Case remarks When 'Absent' Then 1 End) Absent,
     Sum (Case remarks When 'Late' Then 1 End) Late
 FROM attendance 
 GROUP BY id;

从一个表中加入两个select语句

3 个答案: