Sql server® 2012 t-sql fundamentals

The 

READ UNCOMMITTED

 Isolation Level

READ UNCOMMITTED is the lowest available isolation level. In this isolation level, a reader doesn’t 

ask for a shared lock. A reader that doesn’t ask for a shared lock can never be in conflict with a writer 

that is holding an exclusive lock. This means that the reader can read uncommitted changes (also 

known as dirty reads). It also means that the reader won’t interfere with a writer that asks for an ex-

clusive lock. In other words, a writer can change data while a reader that is running under the READ 

UNCOMMITTED isolation level reads data.

To see how an uncommitted read (dirty read) works, open two query windows (I will refer to them 

as Connection 1 and Connection 2). Make sure that in all connections your database context is that of 

the sample database TSQL2012.

Run the following code in Connection 1 to open a transaction, update the unit price of product 2 

by adding 1.00 to its current price (19.00), and then query the product’s row.

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