Queries for migrating data in live database?

Question

I am writing code to migrate data from our live Access database to a new Sql Server database which has a different schema with a reorganized structure. This Sql Server database will be used with a new version of our application in development.

I've been writing migrating code in C# that calls Sql Server and Access and transforms the data as required. I migrated for the first time a table which has entries related to new entries of another table that I have not updated recently, and that caused an error because the record in the corresponding table in SQL Server could not be found

So, my SqlServer productions table has data only up to 1/14/09, and I'm continuing to migrate more tables from Access. So I want to write an update method that can figure out what the new stuff is in Access that hasn't been reflected in Sql Server.

My current idea is to write a query on the SQL side which does SELECT Max(RunDate) FROM ProductionRuns, to give me the latest date in that field in the table. On the Access side, I would write a query that does SELECT * FROM ProductionRuns WHERE RunDate > ?, where the parameter is that max date found in SQL Server, and perform my translation step in code, and then insert the new data in Sql Server.

What I'm wondering is, do I have the syntax right for getting the latest date in that Sql Server table? And is there a better way to do this kind of migration of a live database?

Edit: What I've done is make a copy of the current live database. Which I can then migrate without worrying about changes, then use that to test during development, and then I can migrate the latest data whenever the new database and application go live.

Answer 1

You may want to look at investing in a tool like SQL Data Compare.

I believe it has support for access databases too, and you can download a trial.

Answer 2

I personally would divide the process into two steps.

1. I would create an exact copy of Access DB in SQLServer and copy all the data
2. Copy the data from this temporary SQLServer DB to your destination database

In that way you can write set of SQL code to accomplish second step task

Alternatively use SSIS

Answer 3

Generally when you convert data to a new database that will take it's place in porduction, you shut out all users of the database for a period of time, run the migration and turn on the new database. This ensures no changes to the data are made while doing the conversion. Of course I never would have done this using c# either. Data migration is a database task and should have been done in SSIS (or DTS if you have an older version of SQL Server).

If the databse you are converting to is just in development, I would create a backup of the Access database and load the data from there to test the data loading process and to get the data in so you can do the application development. Then when it is time to do the real load, you just close down the real database to users and use it to load from. If you are trying to keep both in synch wile you develop, well I wouldn't do that but if you must, make a nightly backup of the file and load first thing in the morning using your process.

Answer 4

I you are happy with you C# code, but it fails because of the constraints in your destination database you temporarily can disable them and then enable after you copy the whole lot.

I am assuming that your destination database is brand new DB with no data, and not used by anyone when the transfer happens

Answer 5

It sounds like you have two problems:

1. You're migrating data from one database to another.
2. You're changing your schema.

Doing either of these things is tricky if you are trying to migrate the data while people are using the data.

The simplest approach is to migrate the data based on a static copy of the data, and also to queue updates to that data from the moment you captured the static copy. I don't know how easy this is in Access, but in SQLServer or Oracle you can use the redo logs for this or a manual solution using triggers. The poor-man's way of doing this is to make triggers for all the relevant tables that log the primary key of the records that have changed. Then after the old database is shut off you can iterate over those keys and get those records from the old database and put them into the new database. Just copy the whole record; if the record was deleted then delete it from the new database.

Your problem is compounded by the fact that you can't simply copy the data, you have to transform it. This means you probably have to shut down both databases and re-migrate the records based on the change list. It will take a lot of planning to ensure you get things right and I'd recommend writing a testing script that can validate that the resulting data is correct.

Also I'd ensure that the code for the migration runs inside one of the databases if possible. Otherwise you are copying the data twice and this will significantly harm the performance.