Q

Referencing a renamed column on SELECT clause in SQL

The SQL Standard does not allow a renamed column on the SELECT clause to be referenced on a different part of the SQL statement such as the WHERE clause. To do it on a single statement, an SQL common table expression can be used.

I am beginning to use SQL and wondered if this is possible. How can I define a column in the SELECT statement and use it in the WHERE:
SELECT name, CASE WHEN type = 1 THEN 'A' WHEN type = 2 THEN 'B' ELSE 'C' END AS typeflag FROM custtable WHERE typeflag = 'B'

I know this can be done by doing "where type = 2" but I can't give you an easier example. When I try this, SQL says "typeflag is not a column in the table."

Unfortunately, the SQL Standard does not allow a renamed column (typeflag in your example) on the SELECT clause to be referenced on a different part of the SQL statement such as the WHERE clause. That's why the error message states that typeflag is not found because the WHERE clause doesn't have access to the renamed column name on the SELECT clause.

To do this on a single statement, you can use an SQL common table expression. I find table expressions helpful in breaking a complex SQL statement into smaller, simpler components. Here's an example of implementing your statement with a common table expression:

WITH custexp AS (SELECT name, CASE WHEN type = 1 THEN 'A' WHEN type = 2 THEN 'B' ELSE 'C' END AS typeflag FROM custtable) SELECT name, typeflag FROM custexp WHERE typeflag='B'

This was first published in October 2008

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