The Language of SQL, 3rd Edition

15

Set Logic

Keywords Introduced

UNION • UNION ALL • INTERSECT • EXCEPT

The various joins and subqueries of the previous few chapters have dealt with different ways of combining data from multiple tables. The result, however, has always been a single SELECT statement. Now we’ll extend the concept of combining data in multiple tables to the possibility of combining data from multiple queries. In other words, we’ll look at a way to write a single SQL statement that contains more than one SELECT to retrieve data.

The concept of combining queries is often referred to as set logic, a term taken from mathematics. Each SELECT query can be referred to as a set of data. The set logic we’ll employ and examine in this chapter will address four basic scenarios. ...