A dirty read is an uncommitted read, which may or may not be exist in the table. A phantom is a ghost record that doesn't appear in a transaction initially but appears if it is read again because some other transaction has inserted rows matching the criteria.
Here are examples of both:
--Dirty read example
create table t1 (c1 int null, c2 varchar(50) null)
go
--
insert t1(c1, c2) values (1, 'one')
insert t1(c1, c2) values (2, 'two')
insert t1(c1, c2) values (3, 'three')
insert t1(c1, c2) values (4, 'four')
begin tran
update t1 set c2 = 'zero'
-- let this run in the current query window and open a new query window to run the below statement
--and you will see all 4 rows having a value of 0 in column c2, which is uncommitted data
select * from t1 with (nolock)
--uncomment this and run the below statement and rerun the above statement
--and you will see the previous values of one, two, three and four
--rollback tran
Here's an example of a phantom reads:
--Phantom example
create table t1 (c1 int null, c2 varchar(50) null)
go
--
insert t1(c1, c2) values (1, 'one')
insert t1(c1, c2) values (2, 'two')
insert t1(c1, c2) values (3, 'three')
insert t1(c1, c2) values (4, 'four')
-- let this run in the current first query window and open a second query window to run the below statement
--and you will see all 2 rows having a value in column c2 starting with character t
begin tran
select * from t1
where c2 like 't%'
--now insert the new value of ten (matching the query criteria - starts with t) from the first query window
insert t1(c1, c2) values (10, 'ten')
--Run the below statement again from the second query window that is open and you will see the new row
--that got inserted - so 3 rows are seen including the newly inserted ten
--this new row is a phantom read
select * from t1
where c2 like 't%'
--uncomment this and run the below statement in the second query window
--rollback tran
Satish Kartan www.sqlfood.com
