Answer / michael

Advantage:
applying nolock in select statement will increase
concurreny and performance in fetching.

Disadvantage:
it does'nt consider the transaction scenario, even if the
transaction is under process not yet commited or rollback
it will fetch the current record
eg

begin tran
insert into tablename
values('1','asdf')

select * from tablename with (nolock)

-- it will display the table with inserted record.
but the transaction is not completed, it can be rollback
so it became dirty read.

Answer / naren

Using NOLOCK politely asks SQL Server to ignore locks and
read directly from the tables. This means you completely
circumvent the lock system, which is a major performance
and scalability improvement. However, you also completely
circumvent the lock system, which means your code is living
dangerously. You might read the not-necessarily-valid
uncommitted modifications of a running transaction. This is
a calculated risk.

Answer / guest

Do not issue shared locks and do not honor exclusive locks.
When this option is in effect, it is possible to read an
uncommitted transaction or a set of pages that are rolled
back in the middle of a read. Dirty reads are possible. Only
applies to the SELECT statement.

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