UPDATE t1

SET Id = COALESCE(t2a.Id,t2b.Id,t2c.Id)

FROM Table1 t1

LEFT OUTER JOIN Table2 t2a ON t2a.Acc = t1.AccType1

LEFT OUTER JOIN Table2 t2b ON t2b.Acc = t1.AccType2

LEFT OUTER JOIN Table2 t2c ON t2c.Acc = t1.AccType3

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Deja View - The strange feeling that somewhere, sometime you've optimised this query before

How to get the best help on a forum

http://www.sqlservercentral.com/articles/Best+Practices/61537