a key that can be used to uniquely identify a row in a table is called a primary key. keys are commonly used to join or combine data from two or more tables. for example, an "employee" table may contain a column named "location" which contains a value that matches the key of a "location" table. keys are also critical in the creation of indexes, which facilitate fast retrieval of data from large tables. any column can be a key, or multiple columns can be grouped together into a compound key. it is not necessary to define all the keys in advance; a column can be used as a key even if it was not originally intended to be one.