What is a UNION and how is it different from UNION ALL?

< 1 min read

UNION combines results from two queries and removes duplicate rows automatically.
UNION ALL also combines results but keeps duplicates, so it’s faster.
In one project, I merged customer data from website and mobile app tables.
Using UNION removed duplicate users who registered on both platforms.
But when analyzing total login activity, I used UNION ALL to keep all records.
UNION performs a distinct/sort operation → slightly slower.
UNION ALL skips duplicate check → better performance for large datasets.
So I use UNION for unique reporting and UNION ALL for transactional analysis.

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