問題描述
為什么需要在 HAVING 之后而不是 WHERE 中放置自己創建的列(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?
與執行 WHERE 1(編寫整個定義而不是列名)相比，是否有任何缺點?
And are there any downsides instead of doing WHERE 1 (writing the whole definition instead of a column name)?
推薦答案
為什么需要在 HAVING 之后放置自己創建的列(例如選擇 1 作為數字")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 之前應用，HAVING 在之后應用(并且可以過濾聚合).
WHERE is applied before GROUP BY, HAVING is applied after (and can filter on aggregates).
通常，您不能在這兩個子句中引用別名，但是 MySQL 允許在 GROUP BY、 中引用 和 SELECT 級別的別名>ORDER BYHAVING.
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.
與執行WHERE 1"(編寫整個定義而不是列名)相比，是否有任何缺點
And are there any downsides instead of doing "WHERE 1" (writing the whole definition instead of a column name)
如果您的計算表達式不包含任何聚合，則將其放入 WHERE 子句中很可能會更有效.
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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