SQL Interview: #14: Set operations using EXCEPT

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: Medium

Question:

UNION and UNION ALL are commonly used to combine two sets of rows into a single set of rows.

EXCEPT is another set operator.

Can you explain what it does?

Answer:

EXCEPT is used to remove any rows in the first set of rows, if the same rows appear in the second set.

For example, in the code below:

The query returns all the Trading Names for customers unless a supplier also has that same name.

In other database engines (e.g. Oracle), this operator is called MINUS.