This could be done in a possibly more general fashion using DelimitedSplit8K but if you've only got 1 or 2 hyphens, you can also do it like this:
WITH SampleData (OrderNo) AS
SELECT 'SO-123456' AS OrderNo
UNION ALL SELECT 'SO-123456-01'
UNION ALL SELECT 'SO-123456-2'
UNION ALL SELECT 'SO-123457'
UNION ALL SELECT 'SO-123457-1'
UNION ALL SELECT 'SO-123457-02'
UNION ALL SELECT 'SO-123458'
SELECT OrderNo, NewOrderNo
FROM SampleData a
SELECT p1=CHARINDEX('-', OrderNo)
,s1=RIGHT(OrderNo, LEN(OrderNo)-CHARINDEX('-', OrderNo))
SELECT NewOrderNo=LEFT(OrderNo, p1+CHARINDEX('-', s1+'-')-1)
Edit: Note that this also works if there are more than two characters ahead of the first hyphen.
My mantra: No loops! No CURSORs! No RBAR! Hoo-uh![/I]
My thought question: Have you ever been told that your query runs too fast?
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.
Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]