Minimum transaction isolation level to avoid “Lost Updates”

Question

With SQL Server\'s transaction isolation levels, you can avoid certain unwanted concurrency issues, like dirty reads and so forth.

The one I\'m interested in right n

Answer 1

The example in the book is of Clerk A and Clerk B receiving shipments of Widgets.

They both check the current inventory, see 25 is in stock. Clerk A has 50 widgets and updates to 75, Clerk B has 20 widgets and so updates to 45 overwriting the previous update.

I assume she meant this phenomena can be avoided at all isolation levels by Clerk A doing

UPDATE Widgets
SET StockLevel = StockLevel + 50
WHERE ...

and Clerk B doing

UPDATE Widgets
SET StockLevel = StockLevel + 20
WHERE ...

Certainly if the SELECT and UPDATE are done as separate operations you would need repeatable read to avoid this so the S lock on the row is held for the duration of the transaction (which would lead to deadlock in this scenario)