-- step 1
FROM (
  x
  JOIN LEFT y
  ON x.b = y.b
)

-- step 2
FROM (
  y
  JOIN LEFT z
  ON y.c = z.c
)

-- combined ?
FROM (
  -- step 1
  (
    x
    JOIN LEFT y
    ON x.b = y.b
  )
  -- step 2
  JOIN LEFT z
  ON ??.b = z.b
)