Leaving an alias in a group by statement

Question

From time to time if I have used a case statement in my select, and then copy this into the group by, I leave the alias in by mistake.
The alias is essentially superfluous at this point, and is not allowed as it is incorrect syntax.
Why is this not allowed as I cannot see that it affects the overall output? Is it simply a design choice?

Answer 1

这是因为GROUP BY首先在SELECT之前执行，因此尚未定义别名，因此SQL还不知道它。这样，您就可以在ORDER BY中使用别名。

请参阅

下面的SQL执行层次结构

1. FROM
2. ON
3. OUTER
4. WHERE
5. GROUP BY
6. CUBE | ROLLUP
7. HAVING
8. 选择
9. DISTINCT
10. ORDER BY
11. TOP