How to check if a set of rows already exist in the database and skip migrate them? - sql

Question

I need to create a package to migrate a large amount of data from a database table into a different database table. The source table will continuously have new data in like 4,5 days so I will run my package again and again.
I need to migrate all data from this table to another table but I don't want to migrate those data that I already migrated. What kind of transformation I need to use or what SQL command I need to write to do this?

Answer 1

The usual way this is done is by having "audit" timestamps on the source table and migating only records updated or inserted after the last migration.
for example:
Table Sales
sale_id
sale_date
sale_amount
...............
dw_create_date
dw_update_date
Your source extraction could be something along the lines of..
select sales.sale_id,
sales.sale_date,
....
from sales
where dw_updated_date > {last_migration_date}
last_migration_date is usually read from a config file or table.
Other approaches
There are a few other approaches that you could use, but all of these have bigger performance problems as your data size grows.
1) Do a (target-source) data, to get changed rows in the souurce.
select *
from source
minus
select * from target
You could do the same using a join between source and target.
select source.*
from src
left join tgt on (src.id=tgt.id)
where (src.column1 <> tgt.column1 or
src.column2 <> tgt.column2
............
)
Note that either one of these approaches does not take care of deletes in the source. If you want the tables to be in sync, the only way to do that would be do a (source-target) to get insert/update changes and (target-source) to get deleted rows and do the same in the target.
2. Insert and ignore the primary constraint error:
This has serious issues if the data can change in the source and you want the updates propagated to the target. You'd also be querying the entire source each time. It is usually better to use Merge/Upsert along with filtered source data, instead.

Answer 2

I would assume both tables have some unique identifier, no?
Table A has:
1
2
3
4
You're moving that to Table B, but keeping the data in Table A at the same time, yes?
So you've run your job once. Now Table B has:
1
2
3
4
Table A gets updated. It now has:
1
2
3
4
5
6
7
You run your job again, but you only want to send over 5,6,7.
SELECT *
FROM TableA
LEFT OUTER JOIN TableB ON TableA.ID = TableB.ID
WHERE TableB.ID = NULL.
If you have some sample data it would help. Does this give you a good idea?
See joins: http://i.stack.imgur.com/1UKp7.png