In MySQL, NULL is a special value that represents the absence of any value. A column in a table can contain NULL values if it is defined to allow NULL values.
When a column contains a NULL value, it means that the data for that column is unknown or undefined. For example, if we have a table named
employees with columns
salary, and we insert a new row with a NULL value for the
INSERT INTO employees (id, name, age, salary) VALUES (1, 'John Doe', 30, NULL);
This means that the salary information for John Doe is unknown or not applicable.
We can use the IS NULL and IS NOT NULL operators to test for NULL values in MySQL. For example, to select all employees with a NULL salary, we can use the following SQL query:
SELECT * FROM employees WHERE salary IS NULL;
To select all employees with a non-NULL salary, we can use the following SQL query:
SELECT * FROM employees WHERE salary IS NOT NULL;
It is important to keep in mind that NULL values are not equal to any other value, including other NULL values. This means that the comparison operators such as =, <, >, etc. cannot be used to compare NULL values. Instead, we must use the IS NULL or IS NOT NULL operators to test for NULL values.