Logical Conditions

A logical condition combines the results of two component conditions to produce a single result based on them or to invert the result of a single condition. Table 7-4 lists logical conditions.

Table 7-4 Logical Conditions

Type of Condition	Operation	Examples
NOT	Returns `TRUE` if the following condition is `FALSE`. Returns `FALSE` if it is `TRUE`. If it is `UNKNOWN`, then it remains `UNKNOWN`.	SELECT * FROM employees WHERE NOT (job_id IS NULL); SELECT * FROM employees WHERE NOT (salary BETWEEN 1000 AND 2000);
AND	Returns `TRUE` if both component conditions are `TRUE`. Returns `FALSE` if either is `FALSE`. Otherwise returns `UNKNOWN`.	SELECT * FROM employees WHERE job_id = 'PU_CLERK' AND department_id = 30;
OR	Returns `TRUE` if either component condition is `TRUE`. Returns `FALSE` if both are `FALSE`. Otherwise returns `UNKNOWN`.	SELECT * FROM employees WHERE job_id = 'PU_CLERK' OR department_id = 10;

Table 7-5 shows the result of applying the NOT condition to an expression.

Table 7-5 NOT Truth Table

--	TRUE	FALSE	UNKNOWN
NOT	`FALSE`	`TRUE`	`UNKNOWN`

Table 7-6 shows the results of combining the AND condition to two expressions.

Table 7-6 AND Truth Table

AND	TRUE	FALSE	UNKNOWN
TRUE	`TRUE`	`FALSE`	`UNKNOWN`
FALSE	`FALSE`	`FALSE`	`FALSE`
UNKNOWN	`UNKNOWN`	`FALSE`	`UNKNOWN`

For example, in the WHERE clause of the following SELECT statement, the AND logical condition is used to ensure that only those hired before 1989 and earning more than $2500 a month are returned:

SELECT * FROM employees
WHERE hire_date < TO_DATE('01-JAN-1989', 'DD-MON-YYYY')
   AND salary > 2500;

Table 7-7 shows the results of applying OR to two expressions.

Table 7-7 OR Truth Table

OR	TRUE	FALSE	UNKNOWN
TRUE	`TRUE`	`TRUE`	`TRUE`
FALSE	`TRUE`	`FALSE`	`UNKNOWN`
UNKNOWN	`TRUE`	`UNKNOWN`	`UNKNOWN`

For example, the following query returns employees who have a 40% commission rate or a salary greater than $20,000:

SELECT employee_id FROM employees
   WHERE commission_pct = .4 OR salary > 20000;