Can you use an alias in the WHERE clause in mysql?

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”.

I don't know if this works in mysql, but using sqlserver you can also just wrap it like:

select * from (  -- your original query  select .. sum(reviews.rev_rating)/count(reviews.rev_id) as avg_rating   from ...) Foowhere Foo.avg_rating ...

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Still I would make this clear: The question is actually not about whether alias names can be used in the WHERE clause.

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

is an aggregation. In the WHERE clause we restrict records we want from the tables by looking at their values. sum(reviews.rev_rating) and count(reviews.rev_id), however, are not values we find in a record; they are values we only get after aggregating the records.

So WHERE is inappropriate. We need HAVING, as we want to restrict result rows after aggregation. It can't be

WHERE avg_rating > 10

nor

WHERE sum(reviews.rev_rating) / count(reviews.rev_id) > 10

hence.

HAVING sum(reviews.rev_rating) / count(reviews.rev_id) > 10

on the other hand is possible and complies with the SQL standard. Whereas

HAVING avg_rating > 10

is only possible in MySQL. It is not valid SQL according to the standard, as the SELECT clause is supposed to get executed after HAVING. From the MySQL docs:

Another MySQL extension to standard SQL permits references in the HAVING clause to aliased expressions in the select list.

The MySQL extension permits the use of an alias in the HAVING clause for the aggregated column

https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html