First this clause of your statement:
select s.sno from t,s where s.sid=t.sid and t.sno is null
will return more than a single value, this then yieds the error
Msg 512, Level 16, State 1, Line 1
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, = or when the subquery is used as an expression.
The statement has been terminated.