Find tuples that are in one relation but not in another.
The Set Difference operation, denoted by the symbol −, is a binary operation that, given two union-compatible relations R and S, produces a new relation containing all tuples that are in R but are NOT in S. Just like the Union operation, the two input relations must be union-compatible, meaning they must have the same number of attributes with compatible domains. The operation is not commutative, which means that R − S is not the same as S − R. The first expression gives you records unique to R, while the second gives you records unique to S. This operation is useful for finding exceptions or discrepancies between two sets of data. For example, if we have a 'Students' relation and a 'Students_Who_Passed' relation (both with a 'student_id' attribute), the operation Students − Students_Who_Passed would give us a list of students who did not pass. In SQL, the Set Difference operation can be implemented using the `EXCEPT` or `MINUS` operator (depending on the specific database system) or by using subqueries with the `NOT IN` or `NOT EXISTS` clause. It is a powerful tool for filtering out unwanted records based on their presence in another dataset.