There are only four types of relational keys Candidate Key As stated above, a candidate key is any set of one or more columns whose combined values are unique among all occurrences (i.e., tuples or rows). Since a null value is not guaranteed to be unique, no component of a candidate key is allowed to be null. There can be any number of candidate keys in a table . Relational pundits are not in agreement whether zero candidate keys is acceptable, since that would contradict the (debatable) requirement that there must be a primary key. Primary Key The primary key of any table is any candidate key of that table which the database designer arbitrarily designates as "primary". The primary key may be selected for convenience, comprehension, performance, or any other reasons. It is entirely proper (albeit often inconvenient) to change the selection of primary key to another candidate key. As we will see below, there is no property of a primary key which is not shared by all other candidate keys in of the table except this arbitrary designation. Unfortunately E-R methodologies and RDBMS products have come to rely on the primary key almost to the exclusion of the concept of candidate keys, which are rarely supported by software. Alternate Key The alternate key s of any table are simply those candidate keys which are not currently selected as the primary key. According to { Date95 } (page 115), "... exactly one of those candidate keys [is] chosen as the primary key [and] the remainder, if any, are then called alternate keys ." An alternate key is a function of all candidate keys minus the primary key. Foreign Key A foreign key is a set of one or more columns in any table (not necessarily a candidate key, let alone the primary key, of that table) which may hold the value(s) found in the primary key column(s) of some other table. So we must have a primary key to match the foreign key. .