如何在列上添加检查约束以进行更新

Question

I'm adding a column to a table, and want to set a constraint on it, that it's not allowed to be updated until a given date, stored in the same table row.

This is what I've got so far:

ALTER TABLE sometable 
    ADD somecolumn BIT NOT NULL DEFAULT 0;

ALTER TABLE sometable 
    ADD CONSTRAINT somedate_before_now 
        CHECK (somedatecolumn < GETDATE()); 

How do I get this to work, for updates of somecolumn only? Inserts and updates of other columns should be allowed.

I'm testing this on an in memory H2 database, but it will have to work with SQL Server as well.

Answer 1

I think the safe way to do this is using a trigger.

A constraint such as this:

CHECK (somedatecolumn < GETDATE())

doesn't really make sense from a relational point of view. It can guarantee the values are true on an update or insert . However, the database cannot guarantee that the values are true in general. You should use CHECK constraints for things that you always want to be true in a row.

(The problem is that GETDATE() changes with no changes on the row.)