MS-SQL: unique constraint case-insensitive How do I add a case-insensitive constraint without altering the column collation?

I've tried 

CONSTRAINT [AK_User_Username_UserType] UNIQUE (LOWER([Username]) [UserType])


and

CREATE UNIQUE INDEX [AK_User_Username_UserType] UNIQUE (LOWER([Username]) [UserType])


But it won't compile...

Do I need to add a computed column to achieve this??
Yes a computed column would achieve this for you. Or you could create an trigger that will fire on insert or updates. Here is link to a question like yours on stackoverflow.