In SharePoint 2010 we can now have a greater level of control over the realtionship between a lookup column and the items in it’s source list.
When you create a lookup column, you’ll see the option to create a relationship:
These relationship enforcements are a powerful way to ensure that your list item metadata maintains its referential integrety.
One interesting thing is the behavoir of the Recycle Bin when you have Cascade Delete set on a column. If you delete a source item, it will of course delete other items that refer to that item. If you look in the Recycle Bin, you’ll only see one item – the item from the source list.
Notice the relationship icon next to the item – this lets you know that this item represents an enforced relationship. If you restore this item, it will also restore the related items that it deleted.
A few other things to keep in mind about relationship enforcement:
1. It is not available if you allow Multiple Values on the lookup column.
2. It is not available in Site Columns
3. When you set a relationship, the column will automatically be indexed.
