In the previous chapters on INNER JOIN and OUTER JOIN, we studied how to construct a SELECT statement using the SELECT, FROM, and WHERE clauses. The focus of those two chapters was on constructing complex JOINs within the FROM clause. To construct a UNION, you now have to graduate to a SELECT expression that links two or more SELECT statements with the UNION operator. Each SELECT statement can have as simple or complex a FROM clause as you need to get the job done.
Let’s start simply by creating a UNION of two simple SELECT statements that use a single table in the FROM clause. Figure 10-4 shows the syntax diagram for a UNION of two simple SELECT statements.