T-SQL database design and tables_问答_开发者_运维开发者技术经验分享

I'd like to hear some opinions or discussion on a matter of database design. Me and my colleagues are developing a complex application in finance industry that is being installed in several countries.

Our contractors wanted us to keep a single application for all the countries so we naturally face the difficulties with different workflows in every one of them and try to make the application adjustable to satisfy various needs.

The issue I've encountered today was a request from the head of the IT department from the contractors side that we keep the database model in terms of tables and columns they consist of.

For examlpe, we got a table with different risks and we needed to add a flag column IsSomething (BIT NOT NULL ...). It fully qualifies to exists within the risk table according to the third normal form, no transitive dependency to the key, a non key value 开发者_如何学Go...

BUT, the guy said that he wants to keep the tables as they are so we had to make a new table "riskinfo" and link the data 1:1 to the new column.

What is your opinion ?

We add columns to our tables that are referenced by a variety of apps all the time.

So long as the applications specifically reference the columns they want to use and you make sure the new fields are either nullable or have a sensible default defined so it doesn't interfere with inserts I don't see any real problem.

That said, if an app does a select * then proceeds to reference the columns by index rather than name you could produce issues in existing code. Personally I have confidence that nothing referencing our database does this because of our coding conventions (That and I suspect the code review process would lynch someone who tried it :P), but if you're not certain then there is at least some small risk to such a change.

In your actual scenario I'd go back to the contractor and give your reasons you don't think the change will cause any problems and ask the rationale behind their choice. Maybe they have some application-specific wisdom behind their suggestion, maybe just paranoia from dealing with other companies that change the database structure in ways that aren't backwards-compatible, or maybe it's just a policy at their company that got rubber-stamped long ago and nobody's challenged. Till you ask you never know.

This question is indeed subjective like what Binary Worrier commented. I do not have an answer nor any suggestion. Just sharing my 2 cents.

Do you know the rationale for those decisions? Sometimes good designs are compromised for the sake of not breaking currently working applications or simply for the fact that too much has been done based on the previous one. It could also be many other non-technical reasons.

Very often, the programming community is unreasonably concerned about the ripple effect that results from redefining tables. Usually, this is a result of failure to understand data independence, and failure to guard the data independence of their operations on the data. Occasionally, the original database designer is at fault.

Most object oriented programmers understand encapsulation better than I do. But these same experts typically don't understand squat about data independence. And anyone who has learned how to operate on an SQL database, but never learned the concept of data independence is dangerously ignorant. The superficial aspects of data independence can be learned in about five minutes. But to really learn it takes time and effort.

Other responders have mentioned queries that use "select *". A select with a wildcard is more data dependent than the same select that lists the names of all the columns in the table. This is just one example among dozens.

The thing is, both data independence and encapsulation pursue the same goal: containing the unintended consequences of a change in the model.

Here's how to keep your IT chief happy. Define a new table with a new name that contains all the columns from the old table, and also all the additional columns that are now necessary. Create a view, with the same name as the old table, that contains precisely the same columns, and in the same order, that the old table had. Typically, this view will show all the rows in the old table, and the old PK will still guarantee uniqueness.

Once in a while, this will fail to meet all of the IT chief's needs. And if the IT chief is really saying "I don't understand databases; so don't change anything" then you are up the creek until the IT chief changes or gets changed.