Mastering Oracle SQL

WHERE Clause Evaluation

Now that we have seen the WHERE clause in action, let’s take a look at how it is evaluated. As we mentioned, the WHERE clause consists of one or more conditions that evaluate independently to TRUE or FALSE. If your WHERE clause consists of multiple conditions, the conditions are separated by the logical operators AND and OR. Depending on the outcome of the individual conditions and the placement of these logical operators, Oracle will assign a final value of TRUE or FALSE to each candidate row, thereby determining whether a row will be included in the final result set.

Let’s look at the ‘Acme Industries’ query again:

SELECT p.part_nbr, p.name, p.supplier_id, p.status, p.inventory_qty,
  s.supplier_id, s.name
FROM part p, supplier s
WHERE s.supplier_id = p.supplier_id
  AND s.name = 'Acme Industries';

The WHERE clause consists of two conditions separated by AND. Thus, a row will only be included if both conditions evaluate to TRUE. Table 2-1 shows the possible scenarios when conditions are replaced by their possible outcomes.

Table 2-1. Multiple-condition evaluation using AND

Intermediate result	Final result
WHERE TRUE AND TRUE	TRUE
WHERE FALSE AND FALSE	FALSE
WHERE FALSE AND TRUE	FALSE
WHERE TRUE AND FALSE	FALSE

Using basic logic rules, we can see that the only combination of outcomes that results in a final value of TRUE being assigned to a candidate row is where both conditions evaluate to TRUE. Table 2-2 demonstrates the possible outcomes if our ...