SQL UNION ALL Function
Quick summary: The SQL UNION ALL operator combines result sets and keeps duplicate rows.
SQL UNION ALL Syntax
SELECT_statement1 UNION ALL SELECT_statement2SQL
SQL UNION ALL Basic examples
SELECT 1 UNION ALL SELECT 1;SQL
Output:
1, 1Keeps duplicate rows.
SQL UNION ALL Real-world usage
SELECT id FROM logs_2025 UNION ALL SELECT id FROM logs_2026;SQL
Output:
All log IDsCombines partitioned tables efficiently.
SQL UNION ALL Edge cases
SELECT NULL UNION ALL SELECT NULL;SQL
Output:
NULL, NULLNULL values are preserved.
SQL UNION ALL Common mistakes
Using UNION ALL when uniqueness is required
Duplicates are not removed.
Incorrect
UNION ALLCorrect
UNIONChoose based on uniqueness needs.
SQL UNION ALL Frequently Asked Questions
What does UNION ALL do in SQL?
Combines results without removing duplicates.
Use case of UNION ALL?
Faster merging of datasets.
Difference from UNION?
Does not remove duplicates.
Performance?
Faster than UNION.
Common mistake?
Expecting unique results.
Column count must match?
Yes.
Order guaranteed?
No.
Used in ETL?
Yes.
Supports ORDER BY?
Yes (final query).
Handles NULL?
Yes.
Used in analytics?
Yes.
Best practice?
Prefer when duplicates are allowed.