If you try to use a column alias in a where statement, you recieve an error message that reads
"Invalid column name 'myAlias'."
For example, this statement will give an error
SELECT id, CASE WHEN 1=1 THEN 1 END AS myAlias FROM myTable WHERE myAlias = 1
There are two common solutions to this problem – repeating the alias statement in the WHERE using a sub-query (prefered method).
Using a sub-query (prefered method)
SELECT id, myAlias FROM ( SELECT id, CASE WHEN 1=1 THEN 1 END AS myAlias FROM myTable ) data WHERE myAlias = 1
Repeating the alias statement
In this example, the CASE statement is extremely simplified (1=1) – however your example will most likely be much more complicated.
Repeating the statement is not recommended because of the performance overhead.
SELECT id, CASE WHEN 1=1 THEN 1 END AS myAlias FROM myTable WHERE 1=1