哪里 vs 有-数据库问题

WHERE vs HAVING(哪里 vs 有)

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问题描述

为什么需要在 HAVING 之后而不是 WHERE 中放置自己创建的列(例如 select 1 as "number")MySQL?

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

与执行 WHERE 1(编写整个定义而不是列名)相比，是否有任何缺点?

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 中的 WHERE 中?

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?

WHERE 在 GROUP BY 之前应用，HAVING 在之后应用(并且可以过滤聚合).

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引用别名，但是 MySQL 允许在 GROUP BY、 中引用 SELECT 级别的别名>ORDER BY 和 HAVING.

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.

与执行WHERE 1"(编写整个定义而不是列名)相比，是否有任何缺点

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

如果您的计算表达式不包含任何聚合，则将其放入 WHERE 子句中很可能会更有效.

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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