NOLOCk is the bassically a technique which is useful something for the busy system . Its help on the system which is generally consider good practice to improve the concurrency, When in SELECT statement use whith the NOLOCK hints,when NOLOCK hint is taken then Data is read The result is a Dirty Read, which means that another process could be updating the data at the exact time we are reading it. its not a garunteed for the most of data to be read.in the hints give us a advantages of the performance is that our reading of data will not block updates from taking place, and updates will not block our reading of data. SELECT statements take Shared Read lock, This means that multiple SELECT statements are allowed simultaneous access, But other processes are blocked from modifying the data. The updates will queue until all the readshave completed, and reads requested after the update will wait for the updates to complete. The result to our system is delay blocking.