Question

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 

Answer 1

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

 HAVING avg_rating>5 

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

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

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.

From the MySQL Manual:

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”.