How to scan for differences between two queries? - sql

Question

I have a table that loads new data every day and another table that contains a history of changes to that table. What's the best way to check if any of the data have changed since the last time data was loaded?
For example, I have table #a with some strategies for different countries and table #b tracks the changes made to table #a. I can use a checksum() to hash the fields that can change, and add them to the table if the existing hash is different from the new hash. However, MSDN doesn't think this is a good idea since "collisions" can occur, e.g. two different values map to the same checksum.
MSDN link for checksum
http://msdn.microsoft.com/en-us/library/aa258245(v=SQL.80).aspx
Sample code:
declare #a table
(
ownerid bigint
,Strategy varchar(50)
,country char(3)
)
insert into #a
select 1,'Long','USA'
insert into #a
select 2,'Short','CAN'
insert into #a
select 3,'Neutral','AUS'
declare #b table
(
Lastupdated datetime
,ownerid bigint
,Strategy varchar(50)
,country char(3)
)
insert into #b
(
Lastupdated
,ownerid
,strategy
,country
)
select
getdate()
,a.ownerid
,a.strategy
,a.country
from #a a left join #b b
on a.ownerid=b.ownerid
where
b.ownerid is null
select * from #b
--get a different timestamp
waitfor delay '00:00:00.1'
--change source data
update #a
set strategy='Short'
where ownerid=1
--add newly changed data into
insert into #b
select
getdate()
,a.ownerid
,a.strategy
,a.country
from
(select *,checksum(strategy,country) as hashval from #a) a
left join
(select *,checksum(strategy,country) as hashval from #b) b
on a.ownerid=b.ownerid
where
a.hashval<>b.hashval
select * from #b

Answer 1

How about writing a query using EXCEPT? Just write queries for both tables and then add EXCEPT between them:
(SELECT * FROM table_new) EXCEPT (SELECT * FROM table_old)
The result will be the entries in table_new that aren't in table_old (i.e. that have been updated or inserted).
Note: To get rows recently deleted from table_old, you can reverse the order of the queries.

Answer 2

There is no need to check for changes if you use a different approach to the problem.
On your master table create a trigger for INSERT, UPDATE and DELETE which tracks the changes for you by writing to table #b.
If you search the internet for "SQL audit table" you will find many pages describing the process, for example: Adding simple trigger-based auditing to your SQL Server database

Answer 3

Thanks to #newenglander I was able to use EXCEPT to find the changed row. As #Tony said, I'm not sure how multiple changes will work, but here's the same sample code reworked to use Except instead of CHECKSUM
declare #a table
(
ownerid bigint
,Strategy varchar(50)
,country char(3)
)
insert into #a
select 1,'Long','USA'
insert into #a
select 2,'Short','CAN'
insert into #a
select 3,'Neutral','AUS'
declare #b table
(
Lastupdated datetime
,ownerid bigint
,Strategy varchar(50)
,country char(3)
)
insert into #b
(
Lastupdated
,ownerid
,strategy
,country
)
select
getdate()
,a.ownerid
,a.strategy
,a.country
from #a a left join #b b
on a.ownerid=b.ownerid
where
b.ownerid is null
select * from #b
--get a different timestamp
waitfor delay '00:00:00.1'
--change source data
update #a
set strategy='Short'
where ownerid=1
--add newly changed data using EXCEPT
insert into #b
select getdate(),
ownerid,
strategy,
country
from
(
(
select
ownerid
,strategy
,country
from #a changedtable
)
EXCEPT
(
select
ownerid
,strategy
,country
from #b historicaltable
)
) x
select * from #b