Update locks are compatible with shared locks, exclusive are not. An update first takes update locks on all of the rows it need to update and then, once it has all of the rows locked with an update lock, converts that lock to exclusive so that the change can be made.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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