SQL Server Interview Question #78

What is UNION?

Joins, Subqueries, CTEs & Set Operators Mid-Level Intermediate

Detailed Explanation

UNION combines the results of two or more compatible SELECT statements and removes duplicate rows from the combined result.

Each SELECT must return the same number of columns, and corresponding columns must have compatible data types. Because duplicate elimination can require additional work, UNION ALL is preferable when duplicates do not need to be removed.

Code Example

SELECT Email
FROM dbo.Customers
UNION
SELECT Email
FROM dbo.Suppliers;