How can you use the NOT operator in SQL?

In SQL, the NOT operator is used to negate or reverse the result of a condition in a WHERE clause. It is used to filter rows where a specified condition is not met. The NOT operator is a powerful tool for querying databases as it allows you to select rows that do not match a particular condition. Here's how you can use the NOT operator in SQL:

1. Using NOT with Simple Conditions:

   You can use the NOT operator to negate a simple condition. For example:

   SELECT *
   FROM table_name
   WHERE NOT condition;
   

   Example:

   SELECT *
   FROM employees
   WHERE NOT department = 'HR';
   

   This SQL statement would retrieve all employees who do not work in the HR department.

2. Using NOT with Complex Conditions:

   You can also use the NOT operator with complex conditions involving logical operators like AND and OR. For example:

   SELECT *
   FROM table_name
   WHERE NOT (condition1 AND condition2);
   

   Example:

   SELECT *
   FROM products
   WHERE NOT (category = 'Electronics' AND price > 100);
   

   This SQL statement would retrieve products that are not in the 'Electronics' category or have a price less than or equal to 100.

3. Using NOT with NULL Values:

   The NOT operator can also be used to check for non-null values using the IS NULL condition. For example:

   SELECT *
   FROM table_name
   WHERE column_name IS NOT NULL;
   

   This SQL statement would retrieve rows where the specified column does not contain null values.

4. Using NOT with IN or EXISTS:

   You can use the NOT operator to filter rows that do not meet a condition specified in a subquery. For example:

   SELECT *
   FROM customers
   WHERE NOT EXISTS (SELECT * FROM orders WHERE orders.customer_id = customers.customer_id);
   

   This SQL statement would retrieve customers who have not placed any orders.

The NOT operator is versatile and can be used with a wide range of conditions to exclude rows that do not meet the specified criteria. It's a valuable tool for creating complex queries and obtaining the exact data you need from a database.