What are the Different types of Keys in RDBMS ?With example..

Question

difference is very good to know?

Answer 1

From here and here: (after i googled your title)

• Alternate key - An alternate key is any candidate key which is not selected to be the primary key
• Candidate key - A candidate key is a field or combination of fields that can act as a primary key field for that table to uniquely identify each record in that table.
• Compound key - compound key (also called a composite key or concatenated key) is a key that consists of 2 or more attributes.
• Primary key - a primary key is a value that can be used to identify a unique row in a table. Attributes are associated with it. Examples of primary keys are Social Security numbers (associated to a specific person) or ISBNs (associated to a specific book). In the relational model of data, a primary key is a candidate key chosen as the main method of uniquely identifying a tuple in a relation.
• Superkey - A superkey is defined in the relational model as a set of attributes of a relation variable (relvar) for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. Equivalently a superkey can also be defined as a set of attributes of a relvar upon which all attributes of the relvar are functionally dependent.
• Foreign key - a foreign key (FK) is a field or group of fields in a database record that points to a key field or group of fields forming a key of another database record in some (usually different) table. Usually a foreign key in one table refers to the primary key (PK) of another table. This way references can be made to link information together and it is an essential part of database normalization

Answer 2

Ólafur forgot the surrogate key:

A surrogate key in a database is a unique identifier for either an entity in the modeled world or an object in the database. The surrogate key is not derived from application data.