DECLARE
  l_cnt NUMBER;
BEGIN
  SELECT COUNT(*) INTO l_cnt 
    FROM dba_tab_columns
   WHERE owner = 'my_owner'
     AND table_name = 'my_table' AND column_name = 'my_column';
  IF( l_cnt = 1 ) THEN
    EXECUTE IMMEDIATE 'ALTER TABLE my_table DROP COLUMN my_column';
  END IF;
END;