You might want to try comp.databases.mysql for comprehensive answers.
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Geradeaus wrote:
> I have searched but have not yet found how I can resolve the following
> problem :
>
> SELECT adId,
> CASE
> WHEN adId =1 THEN cityId
> WHEN adId =2 THEN regionId
> END AS extraField
> FROM ad
> WHERE extraField <> 0
>
> The problem is "extraField" cannot be used in the WHERE clause... I am using
> mySQL 4.1

....
WHERE adId in (1, 2)

not tested!

> Geradeaus wrote:
>> I have searched but have not yet found how I can resolve the following
>> problem :
>>
>> SELECT adId,
>> CASE
>> WHEN adId =1 THEN cityId
>> WHEN adId =2 THEN regionId
>> END AS extraField
>> FROM ad
>> WHERE extraField <> 0
>>
>> The problem is "extraField" cannot be used in the WHERE clause... I am
>> using
>> mySQL 4.1

This is a documented limitation in MySQL, and in the SQL language.

Excerpts from http://dev.mysql.com/doc/refman/5.0/en/select.html :

A select_expr can be given an alias using AS alias_name.
The alias is used as the expression's column name and can be used
in GROUP BY, ORDER BY, or HAVING clauses.
. . .
It is not allowable to use a column alias in a WHERE clause, because
the column value might not yet be determined when the WHERE clause
is executed.

Example of the latter case:

SELECT SUM(lineitem_value) AS order_subtotal
FROM order_lineitems
WHERE order_subtotal > 999
GROUP BY order_id;

See also http://dev.mysql.com/doc/refman/5.0/...ith-alias.html

Regards,
Bill K.