where not exists

"WHERE NOT EXISTS" is a clause used in SQL queries to filter results based on the absence of a specific condition. It is commonly used in combination with subqueries to check for the non-existence of certain records in a table.

For example, let's say we have two tables: "Customers" and "Orders". We want to find customers who have not placed any orders. We can use the "WHERE NOT EXISTS" clause to achieve this:

SELECT *
FROM Customers c
WHERE NOT EXISTS (
  SELECT 1
  FROM Orders o
  WHERE o.customer_id = c.customer_id
);

In this query, we use a subquery inside the "WHERE NOT EXISTS" clause to check if there are no orders associated with each customer. If the subquery returns no results, it means there are no matching records in the "Orders" table for that particular customer, and they will be included in the final result.