Question

Oh great Stackoverflow, I beseech thee…
I need to do the following:

CREATE UNIQUE INDEX UserName ON Emp(UserName) — Sheesh!

UPDATE Emp SET
UserName = Left(FirstName,1)+LastName

WHERE NOT exists an employee with that UserName already.

Answer 1

This will still allow duplicates… but if you want what you said,
Try this (exactly what you said):

UPDATE Emp SET
   UserName = Left(FirstName,1) + LastName
Where Not Exists (Select * From Emp 
                  Where UserName = Left(FirstName,1) + LastName)

I’d guess, however, that you are trying to avoid duplicates, not just reset UserNames where no existing one is the same as the proposed new value. If so, you should add a unique index on the userName column, but first, run this:

UPDATE Emp SET
   UserName = Left(FirstName,1) + LastName
Where  Not Exists 
             (Select * From Emp 
              Where UserName = Left(FirstName,1) + LastName)
    And Not Exists 
             (Select * From Emp 
              Group By Left(FirstName,1) + LastName
              Having Count(*) > 1)

This will reset all the UserNames that CAN be reset without creating duplicates. (It will not eliminate existing Dupes). If there are no dupes after this, you can apply the index.