How to create UNIQUE constraint in SSMS 2012 [duplicate]
You were almost there. If you use the constraint keyword you need to provide a name.
ALTER TABLE Attendance ADD CONSTRAINT UQ_TagID_SessionID UNIQUE NONCLUSTERED( TagID, SessionID)Or alternatively
ALTER TABLE Attendance ADD UNIQUE NONCLUSTERED( TagID, SessionID)also works but then the constraint is auto named
If you prefer to do it via GUI instead of via ALTER statements, you can also right-click on the table in Object Explorer, select Design, then right-click somewhere on the empty background, and select Indexes/Keys. This will open a dialog where you select "Unique key" as the type.
Doing changes via GUI is a fast way for actions you tend to do rarely and hence are not sure about the syntax.
Many Management Studio dialogs - but not this one, probably as it is a sub dialog of the table designer dialog - have a "Script" button on the top left, which writes the action you are configuring via GUI to a query window so that you can save them for future similar tasks, or a copy and paste them, should you need similar actions.