SQL Interview: 12: Using UNION vs UNION ALL

This is a post in the SQL Interview series. These aren’t trick or gotcha questions, they’re just questions designed to scope out a candidate’s knowledge around SQL Server and Azure SQL Database.

Section: Development Level: Intro

Question:

Consider the following code:

SELECT CustomerID, CustomerName 
FROM Sales.Customers
WHERE BusinessCategory = 'Cafe'

UNION

SELECT CustomerID, CustomerName
FROM Sales.Customers
WHERE City = 'Toronto';

A: What would be the difference in results if the query was changed to use a UNION ALL instead of the UNION?

B: What are the performance differences between using a UNION ALL and a UNION in a SELECT statement?

C: How could this query be rewritten without the UNION?

Answer:

A: If the query was changed to use UNION ALL, a customer might be returned more than once. A UNION performs a DISTINCT operation on the results of the query. A UNION ALL does not perform the DISTINCT operation on the results.

B: A UNION invariably involves more work and is slower as it needs to perform a DISTINCT operation on the results.

C: One option would be to rewrite it as follows:

SELECT CustomerID, CustomerName 
FROM Sales.Customers 
WHERE BusinessCategory = 'Cafe' 
OR City = 'Toronto';

2021-04-22