What is a union query in SQL Server?
In SQL Server, a UNION query is a method used to combine the result sets of multiple SELECT statements. By using a UNION query, you can merge the results of two or more SELECT statements into one result set without any duplicate rows. Different data merging methods can be achieved using keywords such as UNION, UNION ALL, INTERSECT, and EXCEPT. Commonly used UNION query methods include:
- UNION: Combining the results of multiple SELECT statements and removing duplicate rows.
- UNION ALL: Combining the results of multiple SELECT statements, including duplicate rows.
- INTERSECT: Returns the rows that appear in both results of two SELECT statements.
- EXCEPT: Return only the rows that appear in the first SELECT statement results and not in the second SELECT statement results.
