What limitations exist when using conditions in the WHERE clause in SQL?

The WHERE operator in SQL has the following restrictions:

• Conditions must return a boolean value (TRUE, FALSE, or UNKNOWN).
• Aggregate functions (e.g., SUM, COUNT) cannot be used directly in WHERE; for this, HAVING is used.
• Conditions cannot reference aliases defined in SELECT.
• WHERE filters rows before grouping and sorting.
• Some DBMS impose restrictions on the use of subqueries or functions in WHERE.

Example:

SELECT * FROM employees
WHERE salary > 50000 AND department_id = 3;