What is the difference between UNION and UNION ALL?
UNION combines the results of two queries and removes duplicate rows, which requires extra sorting work. UNION ALL combines results and keeps all rows including duplicates, so it is faster. Use UNION when you need unique rows and UNION ALL when duplicates are fine or impossible, because it performs better.
The core difference
Both stack the results of two queries on top of each other. UNION then does the extra work of removing duplicates, while UNION ALL skips that step and returns everything.
SELECT city FROM customers
UNION -- unique cities only
SELECT city FROM suppliers;
SELECT city FROM customers
UNION ALL -- keeps duplicates, faster
SELECT city FROM suppliers;For both, the two queries must return the same number of columns with compatible types.
A smart line is prefer UNION ALL when you know duplicates cannot occur, because it avoids the sort and is faster. Showing you think about performance, not just correctness, stands out.
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