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The order of columns in an UPDATE statement doesn’t matter.
The first column referenced in the statement doesn’t “go first”.
Modify the clause to make sure that a column is updated only once.
What if we use the column twice but in two different ways: Once as a reference, the other as the column to update? SQL Server will run this all at once, using a snapshot of existing data as its source.
And then we'll make use of SQL Server's ability to update Table1 via a derived table definition: update T1 set T1. Date from (select row_number() over(partition by id order by Date) as rowid, id, Date from Table1 where Date is NULL) T1 join (select row_number() over(partition by id order by Date) as rowid, id, Date from Table2) T2 on T1= T2and T1.rowid = T2.rowid; You stated that the order of the matching matters but it seems like you don't have anything to ORDER BY in table 1 to create a guaranteed order to match the other table and there is no way in SQL Server to order the rows after insertion date, because information about that is not stored.
With this in mind it’s not possible to do a matching with the result you want.
The NULL-ing of First Name doesn’t happen before or after we set Last Name to First Name. Try this statement: If SQL Server had to pick an order to update the columns, this flipping of data would fail.
Instead, it sets the Last Name to whatever the First Name happens to be before the update runs.