The EXCEPT operator in SQL is used to return distinct rows from the first query (the left query) that are not present in the second query (the right query). This operator effectively subtracts the results of the right query from the results of the left query.
Here's a basic syntax for using the EXCEPT operator:
SELECT column1, column2, ...
FROM table1
WHERE condition1
EXCEPT
SELECT column1, column2, ...
FROM table2
WHERE condition2;
Key Points to Keep in Mind:
1. Both Queries Must Return the Same Number of Columns: The queries used with EXCEPT must have the same number of columns in the result sets.
2. Data Types Must Match: The data types of the corresponding columns must be compatible.
3. Distinct Rows: EXCEPT automatically yields distinct rows; there is no need to use DISTINCT before the EXCEPT.
Example:
Let’s assume you have two tables: Employees2022 and Employees2023. You want to find all employees who were in the Employees2022 table but not in the Employees2023 table.
Here’s how you could write that query:
SELECT EmployeeID, EmployeeName
FROM Employees2022
EXCEPT
SELECT EmployeeID, EmployeeName
FROM Employees2023;
Explanation:
- The first SELECT statement retrieves the EmployeeID and EmployeeName from the Employees2022 table.
- The EXCEPT operator then looks for all those rows that are not present in the result set of the second SELECT, which retrieves the same columns from the Employees2023 table.
- The final result will include only those employees who are in Employees2022 but not in Employees2023.
Important Note:
Not all databases support the EXCEPT operator (for example, MySQL does not support it directly), so make sure to check the documentation for your specific SQL database system regarding the use of EXCEPT, or you can use alternatives like LEFT JOIN with WHERE IS NULL or NOT EXISTS.