分组依据如何在子查询中工作?

How does group by work in sub queries?

当涉及到 SQL 中的高级聚合函数时,我感到非常不安。我有这个查询:

SELECT Name, avg(Value) Mean, Number
From Table1
where Category = 'Time'
group by Name, Number;

它的作用相当明显。但以防万一,这就是它的样子:

|  Name  |  Mean | Number |
|:------:|:-----:|:------:|
| Thing1 | 112.6 | 1      |
| Thing2 | 111.9 | 1      |
| Thing1 | 109.1 | 2      |
| Thing2 | 99.4  | 2      |
| Thing1 | 91.8  | 3      |
| Thing2 | 80.1  | 3      |
| Thing1 | 73.7  | 4      |
| Thing2 | 47.2  | 4      |

现在我想获得特定数字的这些平均值的总和。

我的想法是做这样的事情:

SELECT SUM(Mean) Total Mean, Number 
FROM (SELECT Name, avg(Value) Mean, Number
    FROM Table1
    WHERE Category = 'Time'
    GROUP BY Name, Number)
GROUP BY Number;

但是很明显,我得到了一个 Incorrect syntax near the keyword 'group' 错误。实现此目标的最佳方法是什么,以便我得到如下所示的 table:

| Total Mean | Number |
|:----------:|:------:|
| 224.5      | 1      |
| 208.5      | 2      |
| 171.9      | 3      |
| 120.9      | 4      |

您的派生 table 缺少别名。

SELECT SUM(Mean) Total Mean, Number 
FROM (SELECT Name, avg(Value) Mean, Number
    FROM Table1
    WHERE Category = 'Time'
    GROUP BY Name, Number) t --alias for the derived table
GROUP BY Number;