Is it possible to use a Case statement in a sql From clause

Question

Is it possible to use a Case statement in a sql From clause using SQL 2005? For example, I'm trying something like:

SELECT Md5 FROM  CASE     WHEN @ClientType = 'Employee' THEN @Source = 'HR'     WHEN @ClientType = 'Member' THEN  @Source = 'Other' END CASE  WHERE Current = 2; 

Answer 1

I don't believe that's possible. For one thing, query optimizers assume a specific list of table-like things in the FROM clause.

The most simple workaround that I can think of would be a UNION between the two tables:

SELECT  md5 FROM    hr WHERE   @clienttype = 'Employee' AND     current = 2 UNION SELECT  md5 FROM    other WHERE   @clienttype = 'Member' AND     current = 2; 

Only one half of the UNION could be True, given the @clienttype predicate.