EXCEPT in DBMS

The EXCEPT operator in a DBMS is used to return the difference between two SELECT statements. It returns the records from the first SELECT statement that are not present in the second SELECT statement.

Here is the basic syntax of the EXCEPT clause in SQL:

SELECT column_name(s) FROM table1
EXCEPT
SELECT column_name(s) FROM table2;

For example, if you have two tables, Orders and Deliveries, and you want to find all orders that have not been delivered yet (assuming order_id is a common column), you could write:

SELECT order_id FROM Orders
EXCEPT
SELECT order_id FROM Deliveries;

This would return a list of order_ids that appear in the Orders table but not in the Deliveries table.

Here are some key points about the EXCEPT operator:

• The number and order of columns, and the data types in both SELECT statements should match.
• It removes duplicate rows from the result set.
• It only returns records from the first SELECT statement that are not in the second SELECT statement.

Just like the INTERSECT operator, all DBMSs does not support the EXCEPT operator. MySQL doesn't support the EXCEPT operator directly, you can simulate EXCEPT using a combination of LEFT JOIN or NOT EXISTS.

Here's how you might do it with LEFT JOIN:

SELECT column_name(s) FROM table1 
LEFT JOIN table2 
ON table1.column_name = table2.column_name
WHERE table2.column_name IS NULL;

This query will return the rows from table1 where there is no matching row in table2 for the specified column.

For example, if you have two tables, Orders and Deliveries, and you want to find all orders that have not been delivered yet (assuming order_id is a common column), you could write:

SELECT o.OrderID FROM Orders o 
LEFT JOIN Deliveries d 
ON o.OrderID = d.OrderID 
WHERE d.OrderID IS NULL

This would return a list of OrderIDs that appear in the Orders table but not in the Deliveries table.