Operations on Sets
In Chapter 1, we discussed how Dr. E. F. Codd invented the relational model on which most modern databases and SQL are based. Two branches of mathematics—set theory and first-order predicate logic—formed the foundation of his new model.
After you graduate beyond getting answers from only a single table, you need to learn how to use result sets of information to solve more complex problems. These complex problems usually require using one of the common set operations to link data from two or more tables. Sometimes, you’ll need to get two different result sets from the same table and then combine them to get your answer.
The three most common set operations are as follows.
Intersection— You use this to find the common elements ...
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