SQL Server: What is Read Uncommitted Isolation Level?

The Read Uncommitted Isolation level permits you to read uncommitted data.
The transaction is completed or not it doesn’t matter because It has never issued the share locks and it allows other transactions to modify data that you are reading.

Anytime you can read your data without any locking issues, but sometimes it generates unexpected results because the data returned by the SELECT are kind of dirty data or in a half state only.

Below is a small demonstration to read uncommitted data.

Now test this READ UNCOMMITTED isolation level:

First, create a table with sample data:

Open a new query window or session and executing this script:

During this 15 second delay, Open a new query window or session and try to SELECT this table under READ UNCOMMITED isolation level:

The result is, You can SELECT Uncommitted data.

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