是否可以在 Select 语句中使用 Aggregate 函数而不使用 Group By 子句?
到目前为止，我已经编写了聚合函数，然后是 Group By 子句，以查找基于 SUM、AVG 和其他聚合函数的值.我对 Group By 子句有点困惑.当我们使用聚合函数时，我需要在 Group By 子句中指定哪些列.否则有什么方法可以不使用 Group By 子句来使用聚合函数.
So far I have written Aggregate function followed by Group By clause to find the values based on SUM, AVG and other Aggregate functions. I have a bit confusion in the Group By clause. When we use Aggregate functions what are the columns I need to specify in the Group By clause. Otherwise Is there any way to use Aggregate functions without using Group By clause.
推荐答案
SELECT 子句中所有没有聚合的列都需要在 GROUP BY 中
All columns in the SELECT clause that do not have an aggregate need to be in the GROUP BY
好:
SELECT col1, col2, col3, MAX(col4)
...
GROUP BY col1, col2, col3
也不错:
SELECT col1, col2, col3, MAX(col4)
...
GROUP BY col1, col2, col3, col5, col6
没有其他列 = 不需要 GROUP BY
No other columns = no GROUP BY needed
SELECT MAX(col4)
...
不起作用:
SELECT col1, col2, col3, MAX(col4)
...
GROUP BY col1, col2
毫无意义:
SELECT col1, col2, col3, MAX(col4)
...
GROUP BY col1, col2, col3, MAX(col4)
在没有 GROUP BY 的情况下对其他列进行聚合(MAX 等)是没有意义的，因为查询变得不明确.
Having an aggregate (MAX etc) with other columns without a GROUP BY makes no sense because the query becomes ambiguous.
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