SQL group by part of string

Question

I have following data in my table:

URL         TIME    DATE
--------------------------------------
/x          11      2013-08-01
/x          11      2013-08-01
/pl/        11      2013-08-01
/pl/        11      2013-08-03
/pl/XXX/    11      2013-08-01
/pl/XXX/    11      2013-08-04
/pl/XXX/1   11      2013-08-01
/pl/XXX/2   11      2013-08-01
/pl/YYY/    11      2013-08-01
/pl/YYY/1   11      2013-08-01
/pl/YYY/2   11      2013-08-04
/pl/YYY/3   11      2013-08-04

Is there a way to group by URL up to third slash (/) in SQL Server? Unfortunately there exists record which contains less than three.

Answer 1

One trick to count the number of slashes in a string is:

len(url) - len(replace(url,'/',''))

You can then use charindex three times to find the position of the third slash:

select  BeforeThirdSlash
,       max([date])
from    (
        select  case 
                when len(url) - len(replace(url,'/','')) < 3 then url
                else substring(url, 1, charindex('/', url, charindex('/', 
                         url, charindex('/', url)+1)+1)-1)
                end as BeforeThirdSlash
        ,       *
        from    @t
        ) as SubQueryAlias
group by
        BeforeThirdSlash

Live example at SQL Fiddle.

Answer 2

A simple expression that lets you grab the substring up to the third '/' character is as follows:

case
    when patindex('%/%/%/%', url) = 0 then url
    else left(url,charindex('/',url,charindex('/',url,charindex('/',url)+1)+1))
end

The patindex checks that there are at least three slashes; the left extracts the substring up to and including the third one.

With this expression in hand, writing a group by is simple:

SELECT
    url3, max(tm), max(dt)
FROM (
    SELECT
        CASE
            WHEN patindex('%/%/%/%', url) = 0 THEN url
            ELSE left(url,charindex('/',url,charindex('/',url,charindex('/',url)+1)+1))
        END AS url3
    ,   tm
    ,   dt
    FROM test
) x
GROUP BY url3

Demo on SqlFiddle.