在哪里与拥有 [英] WHERE vs HAVING

问题描述

为什么要在MySQL中将自己创建的列(例如select 1 as "number")放在HAVING之后而不是WHERE之后?

Why do you need to place columns you create yourself (for example select 1 as "number") after HAVING and not WHERE in MySQL?

还有什么弊端而不是做WHERE 1(写整个定义而不是列名)?

And are there any downsides instead of doing WHERE 1 (writing the whole definition instead of a column name)?

推荐答案

为什么需要在HAVING之后放置自己创建的列(例如，选择1作为数字")，而不是在MySQL中放置WHERE?

Why is it that you need to place columns you create yourself (for example "select 1 as number") after HAVING and not WHERE in MySQL?

在GROUP BY之前应用

WHERE，在GROUP BY之后应用HAVING(并且可以对聚合进行过滤).

WHERE is applied before GROUP BY, HAVING is applied after (and can filter on aggregates).

通常，您不能在这两个子句中都引用别名，但是MySQL允许在GROUP BY，ORDER BY和HAVING中引用SELECT级别的别名.

In general, you can reference aliases in neither of these clauses, but MySQL allows referencing SELECT level aliases in GROUP BY, ORDER BY and HAVING.

还有什么缺点，而不是做"WHERE 1"(写整个定义而不是列名)

And are there any downsides instead of doing "WHERE 1" (writing the whole definition instead of a column name)

如果您计算出的表达式不包含任何聚合，将其放入WHERE子句将最有效率.

If your calculated expression does not contain any aggregates, putting it into the WHERE clause will most probably be more efficient.

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