“I beseech those whose piety will permit them reverently to petition, that they will pray for this union.”
—SAM HOUSTON, TEXAS HERO
In Chapter 7, “Thinking in Sets,” I introduced three fundamental set operations—intersection, difference, and union. Chapter 8, “INNER JOINs,” showed how to perform the equivalent of an intersection operation by linking result sets on key values using INNER JOIN. Chapter 9, “OUTER JOINs,” discussed how to ask for a set difference by using an OUTER JOIN and testing for the Null value. This chapter explains how to do the third operation, a UNION.