Handling NULL Values in WHERE and HAVING Clauses in MySQL
In MySQL, NULL represents missing or unknown values. When using WHERE and HAVING clauses, comparisons with NULL must be handled carefully because standard operators like = or > do not work as expected with NULL.
In WHERE clauses, use
IS NULLorIS NOT NULLto filter rows with NULL values, e.g.,WHERE salary IS NULL.Comparisons like
salary = NULLwill always return FALSE because NULL is not equal to any value, including itself.HAVING clauses handle NULL in aggregates according to function rules: for example,
SUM(NULL)ignores NULLs, whileCOUNT(column)counts only non-NULL values.You can combine IS NULL checks with aggregate filters, e.g.,
HAVING SUM(salary) IS NOT NULL AND SUM(salary) > 100000.
Proper handling of NULL values ensures that queries return accurate results and avoid unexpected omissions when filtering or aggregating data.