WHERE column = NULL

WHERE column IS NULL

NULL comparisons always return UNKNOWN. Only IS NULL / IS NOT NULL work.

Never use = NULL or != NULL. Always use IS NULL / IS NOT NULL

High

Returns 0 rows because NULL = NULL is UNKNOWN

salary + bonus (when bonus is NULL)

salary + COALESCE(bonus, 0)

NULL propagates. Any arithmetic with NULL returns NULL.

Use COALESCE to treat NULL as 0 in calculations

High

If bonus is NULL, entire result is NULL

Using COUNT(*) when you mean COUNT(column)

COUNT(*) includes NULLs, COUNT(column) excludes NULLs

COUNT(*) = total rows. COUNT(column) = non-NULL values.

Choose based on what you need: all rows or non-NULL values

Medium

COUNT(*) counts all rows, COUNT(column) counts non-NULL values