Create test data
create table orders (orderid int,tableid int,orderpart int,timetaken int,itemname varchar(50),datetimeentered datetime);
insert into orders values (1,1,1,10,'Pizza','2013-06-20 05:30:33:343 PM');
insert into orders values (1,1,1,10,'Burger','2013-06-20 05:35:33:343 PM');
insert into orders values (1,1,1,10,'Burger','2013-06-20 05:38:33:343 PM');
insert into orders values (1,1,2,20,'Dhosa','2013-06-20 06:03:33:343 PM');
insert into orders values (1,1,2,10,'Juize','2013-06-20 06:03:33:343 PM');
Run the queries
select datediff(minute,
min(datetimeentered),
max(datetimeentered)
) As TimeDifferenceInMinutes from orders t1 where orderid=1 and tableid=1 and orderpart=1
select datediff(minute,
min(datetimeentered),
max(datetimeentered)
) As TimeDifferenceInMinutes from orders t1 where orderid=1 and tableid=1 and orderpart=2
Results
TimeDifferenceInMinutes
8
TimeDifferenceInMinutes
0