Why do we NOLOCK in SQL server with an example

In SQL Server, the NOLOCK hint, also known as the READUNCOMMITTED isolation level, allows a SELECT statement to read data from a table without acquiring shared locks on the data. This means it can potentially read uncommitted changes made by other transactions, which can lead to what's called "dirty reads."

Here's an example:

Let's say you have a table named Employee with columns EmployeeID and EmployeeName.

CREATE TABLE Employee (
    EmployeeID INT,
    EmployeeName VARCHAR(100)
);

INSERT INTO Employee (EmployeeID, EmployeeName)
VALUES (1, 'Alice'), (2, 'Bob'), (3, 'Charlie');

Now, if two transactions are happening concurrently:

Transaction 1:

BEGIN TRANSACTION
UPDATE Employee
SET EmployeeName = 'David'
WHERE EmployeeID = 1;

Transaction 2:

SELECT EmployeeName
FROM Employee WITH (NOLOCK)
WHERE EmployeeID = 1;

If Transaction 2 uses WITH (NOLOCK) when reading the Employee table, it might read the uncommitted change made by Transaction 1 and retrieve 'David' as the EmployeeName for EmployeeID 1. However, if Transaction 1 rolled back the update, Transaction 2 would have obtained inaccurate or non-existent data, resulting in a "dirty read."

Using NOLOCK can be helpful in scenarios where you prioritize reading data speed over strict consistency. However, it's essential to be cautious since it can lead to inconsistent or inaccurate results, especially in critical transactional systems.

Other considerations like potential data inconsistencies, increased chance of reading uncommitted data, and potential performance implications should be weighed before using NOLOCK. In many cases, alternative isolation levels or database design improvements might be more suitable to achieve the desired performance without sacrificing data integrity.