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### Difference

What’s the difference between 21 and 10? If you answered 11, you’re on the right track! A difference operation (sometimes also called subtract, minus, or except) takes one set of values and removes the set of values from a second set. What remains is the set of values in the first set that are not in the second set. (As you’ll see later, EXCEPT is the keyword used in the SQL Standard.)

#### Difference in Set Theory

Difference is another very powerful mathematical tool. As a scientist, you might be interested in finding what’s different about two sets of chemical or physical sample data. For example, a pharmaceutical research chemist might have two compounds that seem to be very similar, but one provides a certain beneficial effect and ...

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