Explain PL/SQL BOOLEAN values and three-valued logic with NULL

PL/SQL BOOLEAN is a real type (unlike SQL, which has no boolean column type) and can hold TRUE, FALSE or NULL. Any comparison with NULL (v = NULL, v <> NULL) evaluates to NULL, and IF treats a NULL condition the same as FALSE — so ELSE runs. Use IS NULL / IS NOT NULL to test for NULL, and be careful that NOT NULL is still NULL, not TRUE.

DECLARE
  v_flag BOOLEAN;               -- starts as NULL
BEGIN
  IF v_flag THEN
    DBMS_OUTPUT.PUT_LINE('true branch');
  ELSE
    DBMS_OUTPUT.PUT_LINE('runs: NULL condition acts like FALSE');
  END IF;

  IF v_flag IS NULL THEN
    DBMS_OUTPUT.PUT_LINE('and this is how you actually test for NULL');
  END IF;
END;
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