2.3 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 conditions had been separated by OR rather then AND.

Table 2-2. Multiple-condition evaluation using OR

Intermediate result	Final result
WHERE TRUE OR TRUE	TRUE
WHERE FALSE OR FALSE	FALSE
WHERE FALSE OR TRUE	TRUE
WHERE TRUE OR FALSE	TRUE

Next, let's spice our query up a bit by including parts supplied by either Acme Industries or Tilton Enterprises:

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'
    OR s.name = 'Tilton Enterprises');

We now have three separate conditions separated by AND and OR with parentheses surrounding two of the conditions. Table 2-3 illustrates the possible outcomes.

Table 2-3. Multiple-condition evaluation using AND and OR

Intermediate result	Final result
WHERE TRUE AND (TRUE OR FALSE)	TRUE
WHERE TRUE AND (FALSE OR TRUE)	TRUE
WHERE TRUE AND (FALSE OR FALSE)	FALSE
WHERE FALSE AND (TRUE OR FALSE)	FALSE
WHERE FALSE AND (FALSE OR TRUE)	FALSE
WHERE FALSE AND (FALSE OR FALSE)	FALSE

Since a particular part cannot be supplied by both Acme Industries and Tilton Enterprises, the intermediate results TRUE AND (TRUE AND TRUE) and FALSE AND (TRUE AND TRUE) were not included in Table 2-3.

To liven things up even more, we can also throw in the NOT operator. The following query returns data for parts supplied by anyone other than Acme Industries or Tilton Enterprises:

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 NOT (s.name = 'Acme Industries'
    OR s.name = 'Tilton Enterprises');

Table 2-4 demonstrates how the addition of the NOT operator changes the outcome.

Table 2-4. Multiple-condition evaluation using AND, OR, and NOT

Intermediate result	Final result
WHERE TRUE AND NOT (TRUE OR FALSE)	FALSE
WHERE TRUE AND NOT (FALSE OR TRUE)	FALSE
WHERE TRUE AND NOT (FALSE OR FALSE)	TRUE
WHERE FALSE AND NOT (TRUE OR FALSE)	FALSE
WHERE FALSE AND NOT (FALSE OR TRUE)	FALSE
WHERE FALSE AND NOT (FALSE OR FALSE)	FALSE

The use of the NOT operator in the previous example is a bit forced; we will see more natural ways of expressing the same logic in later examples.