Answer
UNION removes duplicates, UNION ALL returns all rows including duplicates. UNION requires extra processing.
Quick Answer
UNION combines results from two SELECT statements and removes duplicates (deduplication step). UNION ALL combines results and keeps all rows including duplicates (faster - no dedup). Both queries must have the same number of columns with compatible types. Use UNION ALL unless you specifically need duplicate removal - it's always faster since MySQL skips the sort/dedup step.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ