Keys in Database Management System
- 1. Keys in DBMS By: Ankit Rai
- 2. Key A key is a set of attributes that can identify each tuple uniquely in the given relation. Different Types Of Keys Super key Candidate key Primary key Alternate key Foreign key Composite key Unique key Secondary key Surrogate key
- 3. Super Key A super key is a set of attributes that can identify each tuple uniquely in the given relation. A super key may consists of any number of attributes. Candidate Key A minimal super key is called as a candidate key. The value of candidate key must always be unique and can never be NULL. Primary Key A primary key is a candidate key that the database designer selects while designing the database. The value of primary key must always be unique and can never be NULL.
- 4. Alternate Key Candidate keys that are left unimplemented or unused after implementing the key are called as alternate keys. Foreign Key An attribute ‘X’ is called as a foreign key to some other attribute ‘Y’ when its values dependent on the values of attribute ‘Y’. The relation in which attribute ‘Y’ is present is called as the referenced relation. The relation in which attribute ‘Y’ is present is called as the referencing relation Composite Key A primary key comprising of multiple attributes and not just a single attribute is as a composite key.
- 5. Unique key It is unique for all the records of the table and may have a NULL value. Once assigned, its value can not be changed i.e. it is non-updatable. Surrogate Key It is unique for all the records of the table and can not be NULL. It is updatable. Secondary Key Secondary key is required for the indexing purpose for better and faster searching.