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Nolock

From Understanding the Sql Server Nolock Hint
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https://www.mssqltips.com/sqlservertip/2470/understanding-the-sql-server-nolock-hint/

Notes

What does the SQL Server NOLOCK hint do?

  1. The NOLOCK hint allows SQL to read data from tables by ignoring any locks and therefore not get blocked by other processes.
  2. This can improve query performance by removing the blocks, but introduces the possibility of dirty reads.
  3. Read further to better understand the use of NOLOCK`.

Dirty Reads

The NOLOCK hint is the same as the READUNCOMMITED hint and can be used as follows with the same results.

SELECT * FROM Person.Contact WITH (READUNCOMMITTED)

This allows the reading of data that hasnt even been comitted to the database yet.

Better way to get dirty reads on tables

USE AdventureWorks2016

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED


SELECT *
FROM [Sales].[SalesOrderHeader] soh
JOIN [Sales].[SalesOrderDetail] sod
ON soh.SalesOrderID = sod.SalesOrderID

SET TRANSACTION ISOLATION LEVEL READ COMMITTED