Superkey: a column or combination of columns containing unique values for each row. The combination of every column in a table is always a superkey because rows in a table must be unique.

Referential integrity means that the column values in one table must match column values in a related table.

Candidate key: a minimal superkey. A superkey is minimal if removing any column makes it no longer unique.

Null Value: A special value that represents the absence of an actual value. A null value can mean that the actual value is unknown or does not apply to the given row.

Primary key: a specially designated candidate key. The primary key for a table cannot contain null values.

Foreign key: a column or combination of columns in which the values must match those of the candidate key. A foreign key must have the same data type as its associated candidate key.

Secondary Key: Also known as an alternate key, or in SQL may also be referred to as a key candidate, a secondary key is a data field or location that is used for data searches and retrieval. Also known as an alternate key, or in SQL may also be referred to as a key candidate, a secondary key is a data field or location that is used for data searches and retrieval.