sql - unions

IF customer = 'A' THEN
  SET headers = 'header 1', 'header 2', 'header 3', ''
  SET summary = 'item 1', 'item 2', 'item 3', ''
ELSE
  SET headers = 'header aa', 'header dd', 'header ff', 'header ad'
  SET summary = data_x, data_y, data_a, data_b
END IF

SELECT
    headers
UNION
SELECT
    summary
UNION
SELECT
    '', '', '', ''
UNION
SELECT
    # some big giant join here
END