<p>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:</p>

<pre><code>sum(reviews.rev_rating)/count(reviews.rev_id) as avg_rating
</code></pre>

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”
.