Lookup columns in SharePoint 2010 are used to establish a relationship between list items in one list and related items in another list. Though in Lookup columns are introduced in MOSS 2007, There are significant changes in SharePoint 2010 for lookup columns.
Some of the changes are:
– Now in SharePoint 2010 you can show display other columns from the target list with the lookup columns, which are also know as projected columns.
– Now in SharePoint 2010 lookup columns now support referential integrity, which prevents users from deleting an item in the target list if it is referenced in one or more items in the related list.
– Now you can configure cascade delete behavior which says if an item is deleted in the target list, all the referenced items in the related list are deleted as well.
– There is a limit which specifies the maximum number of lookup columns a query at one time which is also known as List View Lookup Threshold, can be set from Central Administration site -> Manage Web Applications -> General Settings -> Resource
You can also visit this article for more information on Lookup Columns.
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