One will update all the rows in a, and the other will only update matching rows. Depending on your schema, that might be fine, but as others have mentioned, it might trip all of your timestamps.

9 best practices for faster SQL queries Retrieve only the columns you need Use CASE instead of UPDATE for conditional column updates Keep large-table queries to a minimum Pre-stage your data ...

Everyone wants faster database queries, and both SQL developers and DBAs can turn to many time-tested methods to achieve that goal. Unfortunately, no single method is foolproof or ironclad.

Practical .NET Retrieve and Update Entity Framework with Dynamic Queries You don't have to give up on creating dynamic queries just because you're using Entity Framework. Entity SQL and ObjectQuery ...