-- https://docs.oracle.com/en/database/oracle/oracle-database/23/lnpls/plsql-language-fundamentals.html
DECLARE
  x NUMBER := 5;
  y NUMBER := NULL;
BEGIN
  IF x != y THEN  -- yields NULL, not TRUE
    DBMS_OUTPUT.PUT_LINE('x != y');  -- not run
  ELSIF x = y THEN -- also yields NULL
    DBMS_OUTPUT.PUT_LINE('x = y');
  ELSE
    DBMS_OUTPUT.PUT_LINE
      ('Can''t tell if x and y are equal or not.');
  END IF;
END;
/