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Can someone explain the implications of using "with (nolock)" on queries?

Can someone explain the implications of using “with (nolock)” on queries, when you should/shouldn’t use it?

WITH (NOLOCK) is the equivalent of using READ UNCOMMITED as a transaction isolation level. So, you stand the risk of reading an uncommitted row that is subsequently rolled back, i.e. data never made it into the database. So, while it can prevent reads being deadlocked by other operations, it comes with a risk.