Не "самый быстрый способ", но он будет работать.
with extract1 as
(
SELECT substring(data, 1, charindex('/', data)-1) AS data1,
substring(data, charindex('/', data)+1, LEN(data)) AS data
from atable
), extract2
(
SELECT data1,substring(data, 1, charindex('/', data)-1) AS data2,
substring(data, charindex('/', data)+1, LEN(data)) AS data
from extract1
)
SELECT data1,data2,substring(data, 1, charindex('/', data)-1) AS data3,
substring(data, charindex('/', data)+1, LEN(data)) AS data4
from extract2