In MySQL, the IS NOT NULL comparison function is used to filter and retrieve records from a database where a specific column does not contain NULL values. NULL is a special marker used in databases to represent the absence of data in a particular field. The IS NOT NULL condition is often used in conjunction with the SELECT statement to fetch rows that have non-null values in a specified column.

Syntax

The basic syntax of using IS NOT NULL in a SELECT statement is as follows:

SELECT column1, column2, ...
FROM table_name
WHERE column_name IS NOT NULL;

column1, column2, …: The columns you want to retrieve in the result set.
table_name: The name of the table from which you want to fetch data.
column_name: The specific column for which you want to filter out NULL values.

Example

Consider a hypothetical table named employees with columns like employee_id, first_name, last_name, and salary. If you want to retrieve all the employees with non-null salary values, you can use the following query:

SELECT employee_id, first_name, last_name, salary
FROM employees
WHERE salary IS NOT NULL;

This query will return a result set containing only those rows where the salary column has a non-null value.

Additional Notes

The IS NOT NULL condition is commonly used with columns that should not have missing or unknown values, such as primary key columns or fields representing important information.

If you want to retrieve rows with NULL values in a specific column, you can use the IS NULL condition instead.

SELECT column1, column2, ...
FROM table_name
WHERE column_name IS NULL;

Using IS NOT NULL or IS NULL conditions is crucial for maintaining data integrity and ensuring that the retrieved information meets specific criteria in database queries.