SQL UNION combines the result sets of two or more SELECT statements into a single, deduplicated list. Each SELECT must return the same number of columns in compatible data types. Use UNION when you need a vertical mergea0columns stay the same, rows stackacross tables, views, or subqueries.
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.
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.
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.
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.
Place the keyword UNION between two or more complete SELECT statements, ending the series with an optional ORDER BY for the combined set.
Yes. Standard UNION performs a deduplication step similar to SELECT DISTINCT on the combined set. If you must keep duplicates, use UNION ALL instead.
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.
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.
Use UNION ALL when deduplication is unnecessary or harmful to performance. UNION ALL is faster because it skips the costly DISTINCT step.
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.
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.
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.
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.
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.
UNION removes duplicate rows; UNION ALL keeps every row and is faster because it skips de-duplication.
Yes, column names may differ, but the position, count, and data types must align. Alias columns to keep the result readable.
Yes. Columns are matched by position, not name. Ensure each SELECT lists columns in the same logical order.
Add ORDER BY after the last SELECT. It sorts the combined set. Use column position or alias names defined in the first SELECT.
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