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.

StackExchange Database Administrators Q#105012, answer score: 2