What are Different types of keys in DBMS?

Answer 1

* 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.

For Eg:

The table:

Emloyee(Name,Address,Ssn,Employee_Idprimary_key,Phone_ext)

In the above example Ssn no. and employee identity are ccandidate keys.

* 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.

For Eg:

Emloyee(Name,Address,Ssn,Employee_Idprimary_key,Phone_ext)

* 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.

For Eg:

Emloyee(Name,Address,Ssn,Employee_Idprimary_key,Phone_ext)

<Ssn,Name,Address>

<Ssn,Name>

<Ssn>

All the above are super keys.

* 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.

For Eg:

For a Student....

School(Name,Address,Phone,School_Reg_noprimary_key

Answer 2

primary key -- unique and can't be NULL e.g. employee number in employee table

unique key -- unique and can have one null

foreign key -- reference of primary key and can have null e.g. employee number in sales table

Answer 3

primary key :- The attribute or combination of attributes that uniquely identifies a row or record.

/A primary key is a column (or columns) in a table that uniquely identifies the rows in that table.

CUSTOMERS

CustomerNo FirstName LastName 1 Sally Thompson 2 Sally Henderson 3 Harry Henderson 4 Sandra Wellington

For example, in the table above, CustomerNo is the primary key.

Foreign Key:- an attribute or combination of attribute in a table whose value match a

primary key in another table.

Composite key:- A primary key that consists of two or more attributes is known as

composite key

candidate key:- is a column in a table which has the ability to become a primary key.

Alternate Key:- Any of the candidate keys that is not part of the primary key is called an alternate key.

secondary key:- alternate of primary key.

Answer 4

There are following types of keys in Database Management System . Primary key, Surrogate key, Foreign key, candidate key and alternate key. Primary key is one that identifies a record uniquely. Surrogate key is a system generated primary key . Foreign key is a key of other table that can be used to identify a row explicitly. Candidate key refers to combination of key to identify a row uniquely .

A Key in a database management system context is a column or a group of columns in a table that can be used to identify a unique set of data from a table. Keys are used to impose data identity and also referential integrity. Some common keys are

• Primary Key
• Unique Key
• Composite Key
• Foreign Key

Answer 5

There are following types of keys in Database Management System . Primary key, Surrogate key, Foreign key, candidate key and alternate key. Primary key is one that identifies a record uniquely. Surrogate key is a system generated primary key . Foreign key is a key of other table that can be used to identify a row explicitly. Candidate key refers to combination of key to identify a row uniquely .

Answer 6

A Key in a database management system context is a column or a group of columns in a table that can be used to identify a unique set of data from a table. Keys are used to impose data identity and also referential integrity. Some common keys are

• Primary Key
• Unique Key
• Composite Key
• Foreign Key

Answer 7

A key is an attribute that is used to get data from a table