Learn SQL Database Programming

Working with NULL values

NULL is not an empty string or 0; it is unknown or undefined. NULL is not equal to anything, including itself.

To work with NULL values, use the following built-in functions:

• NULLIF compares two expressions and returns NULL if they are equal. If they are not equal, it returns the first expression.
  • SELECT NULLIF(1, 1); returns NULL.
  • SELECT NULLIF(1, 2); returns 1.
• IFNULL returns the specified value if the expression is NULL.
  • SELECT IFNULL(NULL, 'testing'); returns testing.
  • SELECT IFNULL(NULL, NULL); returns NULL.
• ISNULL returns 0 or 1 depending on whether the value is NULL.
  • SELECT ISNULL(NULL); returns 1.
  • SELECT ISNULL('testing'); returns 0.