Complex Boolean expressions

In a complex WHERE clause, Boolean operators combine multiple conditions.

The following example combines a function with a complex qualification:
WHERE year > 95 AND (quarter = 1 OR quarter = 3)

The OR operator combines two functions, equal(quarter,1) and equal(quarter,3). If either is true, the combination is true. The AND operator combines the result of the greaterthan(year,95) with the result of the Boolean OR operator.

If a WHERE clause contains multiple conditions, the database server constructs a qualification descriptor that contains multiple, nested qualification descriptors.

The following figure shows a complex WHERE clause that contains multiple levels of qualifications. At each level, a Boolean operator combines results from two previous qualifications.
Figure 1. Complex WHERE clause
WHERE region = "southwest" AND 
   (balance < 90 OR aged <= 30)
Function nesting and Qualification- descriptor hierarchy for a three-key index represent the structure of the qualification descriptor that corresponds to the WHERE clause in Complex WHERE clause.
Figure 2. Function nesting
AND(equal(region,'southwest'), 
   OR(lessthan(balance,90), lessthanequal(aged,30)))
The qualification descriptors for the preceding expression have a hierarchical relationship, as the following figure shows.
Figure 3. Qualification- descriptor hierarchy for a three-key index