You can read the various
ISOLATION LEVEL on MSDN
Create a sample table on a sample database
CREATE TABLE [dbo].[Table_1]( [Id] [int] IDENTITY(1,1) NOT NULL, [title] [varchar](50) NULL, CONSTRAINT [PK_Table_1] PRIMARY KEY CLUSTERED ( [Id] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY]
Now open a First query editor (on the database) insert the code below, and execute (do not touch the --rollback) in this case you insert a row on DB but do not commit changes.
begin tran INSERT INTO Table_1 values('Title 1') SELECT * FROM [Test].[dbo].[Table_1] --rollback
Now open a Second Query Editor (on the database), insert the code below and execute
begin tran set transaction isolation level READ UNCOMMITTED SELECT * FROM [Test].[dbo].[Table_1]
You may notice that on second editor you can see the newly created row (but not committed) from first transaction. On first editor execute the rollback (select the rollback word and execute).
-- Rollback the first transaction rollback
Execute the query on second editor and you see that the record disappear (phantom read), this occurs because you tell, to the 2nd transaction to get all rows, also the uncommitteds.
This occurs when you change the isolation level with
set transaction isolation level READ UNCOMMITTED