What does UNION do versus UNION ALL?
PICTURE THIS: DATABASE INDEX
Without indexScan every row
With indexJump to keys
CostWrites slower
Simple meaning
UNION deduplicates
WHY — SQL instead of guessing?
Why interviewers care about SQL:
They want a clean
contrast on SQL, not two memorised paragraphs.
Say what changes for
the developer, then one case where picking wrong hurts.
Stay structured
Name the idea, why it exists, then one short example.
Close cleanly
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:
- 1Define it
UNION deduplicates
- 2UNION ALL keeps duplicates
and is cheaper.
- 3I prefer ALL when
duplicates are impossible.
- 4Close with when you
would choose this approach on a real task.
- 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:
Say this line
“I prefer ALL when duplicates are impossible.”
Break into beats
IpreferALLwhenduplicatesare
Speaking order
2987408337471632900
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
UNION deduplicates UNION ALL keeps duplicates and is cheaper.