SQL将公式应用于AVERAGE值

Question

给出伪表：

+-----+---------------------+------+
| tag | data                | read |
+-----+---------------------+------+
| A   | 2013-10-10 15:00:00 | 1345 |
+-----+---------------------+------+
| A   | 2013-10-10 15:15:00 | 3454 |
+-----+---------------------+------+
| A   | 2013-10-10 15:30:00 | 2345 |
+-----+---------------------+------+
| A   | 2013-10-10 15:45:00 | 1132 |
+-----+---------------------+------+
| B   | 2013-10-10 15:00:00 | 6234 |
+-----+---------------------+------+
| B   | 2013-10-10 15:15:00 | 5432 |
+-----+---------------------+------+
| B   | 2013-10-10 15:30:00 | 4563 |
+-----+---------------------+------+
| B   | 2013-10-10 15:45:00 | 5432 |
+-----+---------------------+------+

是否可以仅使用SQL来应用以下等式？

示例：

result=AVG(A)-(AVG(B)+AVG(C))

或

result=AVG(A)+AVG(B)

按日期分组？

Answer 1

应该计算结果

这是SqlFiddle上的演示。

select (AVG(case when tag = 'A' then read end) + AVG(case when tag = 'B' then read end)) 'result', data
from TBL
group by data

Answer 2

您可以分别为每个标签选择一个总和或平均值，然后在每个单独的查询中选择您想要的操作

select (select SUM([read]) from table where tag = 'A') + 
  (select SUM([read]) from table where tag = 'B')