> If someone can give me a good reason why this is so, it will stop it 
> annoying me so much.

>From the MySQL manual for version 4.0.16:

It is not allowed to use a column alias in a WHERE clause, because the column 
value may not yet be determined when the WHERE clause is executed. See section 
A.5.4 Problems with alias.

I didn't check section A.5.4, but it probably gives a more detailed 


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