A key is one or more columns used to identify a row, or to establish a constraint such as uniqueness or referential integrity relationships. When you create or alter a table, you can establish keys which place constraints on the values that can be entered into that table's columns. Establishing a Foreign Key, for example, causes the XDB Server to reject any column values that violate the criteria defined for that Foreign Key constraint. The XDB Server supports three types of keys: primary, candidate and foreign.