Many developers new to SQL ask about the exact behavior difference between union and union all. Understanding this distinction helps you choose the right option for performance and correctness.
This article breaks down union vs union all in sql key differences explained with practical examples, execution behavior, and impact on query results.
| Aspect | UNION | UNION ALL | When to Use |
|---|---|---|---|
| Duplicate Handling | Removes duplicate rows | Keeps all rows including duplicates | Use UNION for distinct results, UNION ALL for raw combined data |
| Performance | Slower due to distinct sort or hash | Faster with no duplicate elimination | Prefer UNION ALL when duplicates are acceptable or expected |
| Result Set Size | Smaller or equal to combined input | Exactly the combined input row count | Choose based on whether you need deduplication |
| Use in YouTube Tutorials | Shown for clean reporting scenarios | Shown for fast data extraction or debugging | Tutorials often compare both to highlight trade-offs |
How Union Removes Duplicates
When you use union, SQL performs a distinct sort or hash operation to eliminate duplicate rows. This ensures each row in the final output is unique across all combined queries.
The engine may use additional memory and CPU to compare rows, which makes union slower on large datasets. If your data is already distinct or duplicates are irrelevant, consider alternatives.
Why Union All Is Faster
Union all appends the result of each query without any duplicate check. Because it skips the sorting or hashing phase, it usually runs significantly faster than union.
In data pipelines or when you intentionally include repeated rows, union all is the preferred choice for performance-critical workloads on YouTube and in production.
Syntax and Practical Examples
The syntax for both operators is straightforward and identical in structure, differing only by the keyword used after select.
Example of union: select region, amount from sales_east union select region, amount from sales_west.
Example of union all: select region, amount from sales_east union all select region, amount from sales_west.
Behavior with Order By and Top
When using order by with union, the sort applies to the final distinct result set, which can increase processing time.
With union all, order by affects the combined rows before any deduplication because there is none, often yielding faster execution.
Applying top or limit clauses to each query before union can control output size and improve responsiveness in dashboards.
Best Practices for Union vs Union All
- Use union all by default unless you explicitly need deduplication
- Add order by only at the final union if you need a sorted global result
- Index columns involved in distinct comparisons when using union
- Test performance on production-like data to choose the right operator
FAQ
Reader questions
Does union all always return more rows than union?
Yes, union all returns at least as many rows as union because it never removes duplicates, while union eliminates them.
Is union all safe to use if my tables have overlapping data?
Yes, it is safe, but you will see repeated rows. Choose union only when you explicitly need distinct results.
Will using union instead of union all affect query performance on large tables?
Yes, union usually reduces performance on large tables due to the extra distinct sorting or hashing step.
Can I combine union and union all in the same SQL statement?
You can structure nested queries where one part uses union and another uses union all, but each operator applies within its own subquery.