通过预定义的数据库列表加强完整性

Question

Apologies for the terribly worded question (will change it once I work out how to define this problem better).

Lets say I have a table in a database called Supplier . A supply can have a vendor id as PK, and country . The supplier must be working on one, and only one project from a set of three pre-defined project names.

Would the best way to capture this data be:

Create a table called Project which has a Project ID as PK and 'Project Name' field. Create another table called SupplierProject which inherits vendor id and Project ID as primary keys (or is that primary and alternative keys?).

I am thinking in that way, if a project name was ever updated you would only need to update one field in the Project table and not every instance of the project name if instead a field was created in Supplier table called Project Name .

It should be noted that multiple companies could be working on the same project at the same time, but they need to be one of the pre-defined projects from the 'list'.

Does this also increase data integrity has can't enter something which is not on the list?

Is this a normal thing to do in this situation, sort of creating pick tables through a many-to-many relationship structure? I am confused because technically the supplier must work on one, and only one project at any time. This structure gives me the pick list but doesn't enforce this rule.

Hope this is making sense and I am not completely missing the point. Any advice on the best way to handle these situations is much appreciated!

Answer 1

In MySQL, you want a Projects table that contains the list of allowed projects.

In the Suppliers table, you want a project id and a foreign key reference.

Something like this:

create table Projects (
    ProjectId int not null auto_increment primary key,
    ProjectName varchar(255) unique
);

create table Suppliers (
    SupplierId int not null auto_increment primary key,
    . . .
    ProjectId int not null,
    . . .
    foreign key ProjectId references Projects(ProjectId)
);

Then populate Projects with the values you are allowing. Each supplier then has to refer to one of those valid values (the not null requires a reference).

In other databases, you could do this with just a check constraint in the Suppliers table. MySQL allows you to write a check constraint , but it doesn't actually do the checking.