There’s a high demand for using cross site lookup fields. Most solutions offered include deploying a solution or writing your own code. There is, however, a simple method for creating a cross site lookup field by using SharePoint’s own GUI.
In the following example I use 2 sites, http://sharepoint/Site with a custom list named “Values”. I added 3 items in the list as sample data:
The other site is a subsite from the previous one, http://sharepoint/Site/Subsite. Here I added a custom list named “Using Lookup”:
Browse back to the main site, in my example http://sharepoint/Site, and click on Site Actions, Site Settings:
On the site settings page navigate to the site columns gallery:
Click on the create button to create a new site column:
Fill in a desired name:
Next, select the custom list “Values” at the properties and whatever you need to lookup:
Click on OK and navigate to your subsite’s custom list, in my example the list “Using Lookup”. Open up the list settings:
At the columns settings, click on “Add from existing site columns”:
Select the “Lookup Values” column and press the add button:
Click OK, navigate to the list “Using Lookup” and add a new item. The lookup values from the topsite are now selectable!
As far as constraints go, they’re the same as every custom site column.