What Is SQL UNION?

What Is SQL UNION?

SQL UNION merges rows from multiple SELECT statements into a single, deduplicated result set, provided each query returns the same column count and compatible data types.

Why Do Developers Use SQL UNION?

SQL UNION lets you stack data from similar tables or queries without creating new tables. It is ideal when you need a consolidated report across monthly partitions, regional tables, or historical and live datasets.

How Does SQL UNION Differ From a JOIN?

UNION performs a vertical mergerows stack under identical columns, while JOIN performs a horizontal merge, expanding columns across matching rows. Use UNION when schemas match and you want more rows; use JOIN when keys relate data and you need more columns.

What Are the Column Requirements for UNION?

Every SELECT in a UNION must return the same number of columns, in the same order, with compatible data types. Numeric with numeric, date with date, and string with string reduce casting overhead and avoid errors.

What Is the Basic UNION Syntax?

Place the keyword UNION between two or more complete SELECT statements, ending the series with an optional ORDER BY for the combined set.

Does UNION Remove Duplicate Rows?

Yes. Standard UNION performs a deduplication step similar to SELECT DISTINCT on the combined set. If you must keep duplicates, use UNION ALL instead.

Can You UNION More Than Two Queries?

UNION chains indefinitely. Simply keep adding SELECT ... UNION SELECT ... until all datasets are included. Ensure every queryincluding the lastfollows the column and data type rules.

How Do ORDER BY and LIMIT Work With UNION?

ORDER BY and LIMIT apply to the entire unioned set when placed after the last SELECT. Wrap each SELECT in parentheses if you need independent ordering before unioning.

When Should You Use UNION ALL?

Use UNION ALL when deduplication is unnecessary or harmful to performance. UNION ALL is faster because it skips the costly DISTINCT step.

What Are Performance Best Practices for UNION?

Keep column lists explicit, not SELECT *. Index filter columns in each SELECT, avoid unnecessary casting, and prefer UNION ALL when duplicates are safe. Push filters into individual queries to minimize data scanned.

How Do You Troubleshoot Column Mismatch Errors?

Count columns in every SELECT and align corresponding positions. Use CAST or CONVERT to reconcile data types. Alias dissimilar column names to a shared name for clarity.

What Are Real-World Use Cases for UNION?

Combine current and historical sales tables, merge user logs split by region, create consolidated views across sharded databases, and assemble test and production metrics for side-by-side analysis.

What Are Alternatives to UNION?

INSERT ... SELECT into a temp table, create a VIEW that contains the UNION logic, or leverage partitioned tables and table inheritance in PostgreSQL to query partitions transparently.

Key Takeaways

SQL UNION stacks compatible queries, removes duplicates, and simplifies cross-table reporting. Respect column count and type rules, use UNION ALL for speed when safe, and add a single ORDER BY at the end for global sorting.