UNION versus UNION ALL: which one should you default to?
PICTURE THIS: DATABASE INDEX
Simple meaning
UNION concatenates result sets and then removes duplicates, which requires a sort or hash.
WHY — SQL instead of guessing?
Why interviewers care about SQL:
contrast on SQL, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
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:
- 1UNION concatenates result sets
and then removes duplicates, which requires a sort or hash.
- 2UNION ALL keeps duplicates
and is cheaper.
- 3If you know the
sets are disjoint, UNION ALL is the usual production choice.
- 4Give an example
One tiny concrete case you can say aloud.
- 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
UNION concatenates result sets and then removes duplicates, which requires a sort or hash. UNION ALL keeps duplicates and is cheaper.