The only values that you can assign to a BOOLEAN variable are TRUE, FALSE, and NULL.
The following code initializes the BOOLEAN variable done to NULL by default.
Then it assigns it the literal value FALSE, compares it to the literal value TRUE, and assigns it the value of a BOOLEAN expression.
SQL> SQL> DECLARE-- from w ww.ja v a 2s . c o m 2 done BOOLEAN; -- Initial value is NULL by default 3 counter NUMBER := 0; 4 BEGIN 5 done := FALSE; -- Assign literal value 6 WHILE done != TRUE -- Compare to literal value 7 LOOP 8 counter := counter + 1; 9 done := (counter > 500); -- Assign value of BOOLEAN expression 10 END LOOP; 11 DBMS_OUTPUT.PUT_LINE ('counter: ' || counter); 12 END; 13 / counter: 501 PL/SQL procedure successfully completed. SQL>