Field Constraints

Hello,
I have created a DB and TABLE but would like to constrain one field to
accept either "YES" or "NO" only. How can I set this up?
I have tried various syntactic constructs of

Prior_Rx varchar(3) NULL CHECK ('Yes', 'No'),
Prior_Rx varchar(3) NULL CHECK (VALUE 'Yes', 'No'),
Prior_Rx varchar(3) NULL CHECK ('Yes', 'No'),
Prior_Rx varchar(3) NULL CONSTRAINT (CHECK (VALUE
'Yes', 'No')),
Prior_Rx varchar(3) NULL CONSTRAINT (CHECK ('Yes',
'No')),
Prior_Rx varchar(3) NULL CONSTRAINT (CHECK (VALUE Yes,
No)),
Prior_Rx varchar(3) NULL CONSTRAINT (CHECK (Yes, No)),

Prior_Rx varchar(3) NULL CONSTRAINT (CHECK (Yes OR
No)),
Prior_Rx varchar(3) NULL CHECK (Yes OR No),
Prior_Rx varchar(3) NULL CHECK ('Yes' OR 'No'),

Obviously they were all wrong, could someone advise?
Thanks

-- Joe --
PS---------------------------------------------------------------------------------------

Just learned that the datatype should be 'bool' in this case

haven't tested it yet

But had I wanted to restrict it to say either mike or john, or even one
of a list say, 'tea', 'cofee', 'chocolate' or 'espresso', how should
that be constructed?