By Michael Nocito , data analyst · Published August 7, 2026 By the end of this page you can predict what any query does when it meets a missing value, which is the skill that separates "my query returned nothing and I don't know why" from a two-second fix. You will know why = NULL matches zero rows, why one NULL can empty an entire NOT IN , and how NULLs quietly move averages, counts, groups, and sort orders. It is about twenty minutes. Here is what to actually do with it. Next time a ...