How do SQL NULLs behave in filters, joins, and aggregates?
PICTURE THIS: 1, 2, 2, 8
Simple meaning
NULL means unknown, so NULL = NULL is not true and WHERE col = NULL filters nothing
WHY — SQL Analytics instead of guessing?
Why interviewers care about SQL Analytics:
question about SQL Analytics.
trade-offs, and what you would actually do on a Data Science project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1NULL means unknown, so
NULL = NULL is not true and WHERE col = NULL filters nothing
- 2Why it exists
you need IS NULL.
- 3Outer joins introduce NULLs
for missing matches, which then drop out of inner-style comparisons.
- 4Aggregates like SUM skip
NULLs, while an all-NULL column can yield NULL rather than zero.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
NULL means unknown, so NULL = NULL is not true and WHERE col = NULL filters nothing you need IS NULL.