Can Sharepoint help me out with relational data? Well, Patrick Tisseghem's blog can give you an answer to that question.
But do I really need to have a custom solution if I want to link one item with multiple items? Let me elaborate on this a bit more.
Case
Each period you might have a performance meeting with your employees and you want to store the conclusion of this meeting in sharepoint and attach that to the employee.
Well a folder / item structure in a document library might help you.
Step 1: Create a New contact content type and base this on the existing folder content type.
Go to Site actions -> Site settings -> site content types -> create and fill in the following.
Step 2: Create new site columns
Create new or add existing fields to your content type. These fields will hold the employee personal information. You might want to place the new fields under a new group rather then an existing one.
Step 3: Create a "performance" document library
This document library will hold your employees (folders) and the performance documents (items) underneath.
Go to site settings -> create -> document library and give it a useful name.
Once the document library is created you need to go to the settings of your new document library (settings - > document library settings) and go to the advanced settings.
Here you need to make the following changes:
- allow management of content types = yes
- Display "New folder" command on the New menu = No

After these changes you have the ability to attach the New contact content type you created in step 1 by clicking on the Add from existing site content types.
Step 4: create a new view
In order to have a nice overview with employees you can create a standard view that contains the employee fields.
In your list settings click "create a new view" -> standard view and select and order the fields you would like to display.
Make sure this is the default view for this particular list.
Step 5: Create a new employee
In your list you can now press new and New contact
Fill in the employee details
Now each folder is an employee and in each employee folder you can store the e.g. performance documents.
