可以在mysql的WHERE子句中使用别名吗? [英] Can you use an alias in the WHERE clause in mysql?

问题描述

我需要在WHERE子句中使用别名，但是它一直告诉我它是一个未知列.有什么办法可以解决这个问题?我需要选择评级高于x的记录.评分是根据以下别名计算的:

I need to use an alias in the WHERE clause, but It keeps telling me that its an unknown column. Is there any way to get around this issue? I need to select records that have a rating higher than x. Rating is calculated as the following alias:

sum(reviews.rev_rating)/count(reviews.rev_id) as avg_rating

推荐答案

您可以使用HAVING子句，该子句可以看到别名，例如

You could use a HAVING clause, which can see the aliases, e.g.

 HAVING avg_rating>5

，但是在where子句中，您需要重复您的表达式，例如

but in a where clause you'll need to repeat your expression, e.g.

 WHERE (sum(reviews.rev_rating)/count(reviews.rev_id))>5

但是！并非所有的表达式都被允许-使用像SUM这样的聚合函数将不起作用，在这种情况下，您将需要使用HAVING子句.

BUT! Not all expressions will be allowed - using an aggregating function like SUM will not work, in which case you'll need to use a HAVING clause.

从 MySQL手册:

不允许引用 WHERE子句中的列别名， 因为列值可能还没有 在WHERE子句时确定 被执行.参见 B.1.5.4节， 列别名问题" .

It is not allowable to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See Section B.1.5.4, "Problems with Column Aliases".

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