SQL Queries for Mere Mortals: A Hands-On Guide to Data Manipulation in SQL, Second Edition

Union

So far we’ve discussed finding the items that are common in two sets (intersection) and the items that are different (difference). The third type of set operation involves adding two sets (union).

Union in Set Theory

Union lets you combine two sets of similar information into one set. As a scientist, you might be interested in combining two sets of chemical or physical sample data. For example, a pharmaceutical research chemist might have two different sets of compounds that seem to provide a certain beneficial effect. The chemist can union the two sets to obtain a single list of all effective compounds.

Let’s take a look at union in action by examining two sets of numbers. The first set of numbers is as follows.

1, 5, 8, 9, 32, 55, ...