Question

Q:

If I have a composite key combined from 4 fields for example, can I update one of them?

I mean can I execute a statement like this:

UPDATE tb 
SET firstCol = '15', secondCol = 'test2' 
WHERE firstCol = '1' AND serial = '2';

Given:

• my table name is: tb
• my fields are: firstCol, secondCol, serial
• my keys are: firstCol , serial

Any suggestions? Did I miss some concept?

thanks.

Answer 1

You may run into problems in updating if you try to make some row into the same values as an existing row. No matter what you do in an update, the unique constraint will still apply.

If you have related tables and have cascade update turned on, you may have locking issues if many records need to be locked. If you do not have cascade update turned on, you may have issues where a PK cannot be changed until you break those relationships and then put them back after manually changing all the related tables to the new value. This task, either way, should only be done in single user mode during non-peak hours.

Personally, if you need to change the PK, the design of your database is fragile and may cause problems in the future. Especially with a multicolumn key. If this is a one-time, rare change, go ahead and work through the issues. Otherwise, it might be time to decide if having a surrogate key as the PK and a unique index on the multi-columns is a better choice. Multicolumn PKs create much larger indexes not only on the main table but the child tables as well, they can create difficult issues when you need to update one of the columns, and they have performance implications for joins. In general I’m not a fan of them. And defintely not if there are some of those columns that will need updating with any frequency (and by that I mean any large update more than once a year – one or two records OK, but if you are running an update as described more often than once a year, you need to revisit the design in my opinion.).