How can I update a SQL table with a numeric counter based on different values in a column

I have a table which contains items for my customers and I need to update each row with a counter that will tell me if this is the first item they purchased, the 5th or the 20th.

My table looks like this (Example)

table1

CustomerID, transactionID, ItemCounter
29248, 5678953, NULL
29248, 5679526, NULL
29248, 5679863, NULL
86136, 6897856, NULL
86136, 6897956, NULL
86136, 6898113, NULL
86136, 6898569, NULL
16253, 3586214, NULL
16253, 3698874, NULL
16253, 3792365, NULL

After the update I'd like the table to look like this

CustomerID, transactionID, ItemCounter
29248, 5678953, 1
29248, 5679526, 2
29248, 5679863, 3
86136, 6897856, 1
86136, 6897956, 2
86136, 6898113, 3
86136, 6898569, 4
16253, 3586214, 1
16253, 3698874, 2
16253, 3792365, 3
16253, 3798956, 4
16253, 3804569, 5

As you can see the "transactionid" counts up for each customer. I could also add columns such as a "transaction date" to help me order the items by customerid,  but the "transactionid" seems to be enough of a determiner.

Assume the best way to do this is using a CURSOR , but I'm not sure how to crawl through the table and do the "increment" based on the transactionid. The table above is an example and the real table will have 100's of "customers" with multiple "transactions" between them so it will contain tens of thousands of rows based on the time frame being searched. The table is actually a #temptable which is created during a stored procedure used to generate a report so speed of updating the table is important. The #temptable is also the result of selecting data from other tables into it. Just wanted to give that background . Thanks
svasilakosAsked:
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Scott PletcherSenior DBACommented:
UPDATE tn
SET ItemCounter = tn_ic.ItemCounter
FROM table_name tn
INNER JOIN (
    SELECT CustomerID, transactionID,
        ROW_NUMBER() OVER(PARTITION BY CustomerID ORDER BY transactionID) AS ItemCounter
    FROM table_name
) AS tn_ic ON
    tn_ic.CustomerID = tn.CustomerID AND
    tn_ic.transactionID = tn.transactionID
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svasilakosAuthor Commented:
Wow. Assume!!   Thanks for that quick response. Solution worked perfectly for me and was just what I wanted. Nice work!!  

-Regards
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Microsoft SQL Server 2008

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