What is nolock in SQL Server

Nolock is nothing but it decrease the concurrency of busy system.

When there is nolock for data then that data can be taken by another transaction.

Reads the dirty page irrespective to the isolation level. (Not in all scenario).

NOLOCK is used in SELECT queries as query hint, it functions the same way as READ UNCOMITTED isolation level where dirty reads are possible. It is highly recommended for all applications where Performance and concurrency is vital and dirty reads takes a backseat. Like in non-banking and web based applications it is much used, and also in internal web applications used in back-office by multiple users. We can use NOLOCK in all non-financial and non-payment related transactions most of the times. Every domain and case needs to be verified.