SQL Server Interview Question #156

What is READ UNCOMMITTED?

Transactions, Locking, Blocking & Deadlocks Senior Advanced

Detailed Explanation

READ UNCOMMITTED is the least restrictive isolation level. It allows a transaction to read data modified by another transaction even when that transaction has not committed.

This can produce dirty reads and other inconsistent observations. NOLOCK is commonly associated with READ UNCOMMITTED behavior for table access, but it is not a general performance solution and does not mean 'no locks of any kind.'

Use it only when the business can tolerate potentially inconsistent data.

Code Example

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

SELECT *
FROM dbo.Orders;