Invalid column name error in WHERE clause, column selected with CASE

Question

I have a (rather complicated) SQL statement where I select data from lots of different tables, and to cope with a bad legacy data structure, I have a couple of custom colum

Answer 1

The only part of the SQL Statement where it is valid to use an alias declared in the SELECT list is the ORDER BY clause. For other parts of the query you just have to repeat the whole CASE expression and trust the optimiser to recognise it is the same.

If you are on SQL2005+ you can use a CTE to avoid this issue which sometimes helps with readability.

WITH YourQuery As
(

 SELECT
     Limit, 
     Percentage,
     CASE channel
         WHEN 1 THEN channel_1
         WHEN 2 THEN channel_2
         ...
         ELSE 0
     END AS ChannelValue,
     CASE channelu
         WHEN 1 THEN channelu_1
         WHEN 2 THEN channelu_2
         ...
         ELSE '0'
     END AS ChannelWithUnit,
     ...
 FROM 
)

select ...
FROM YourQuery WHERE
ChannelValue > Limit * Percentage / 100