Name the different types of keys? (Topic: Introduction to DBMS)
Answers
Types of SQL Keys
We have following types of keys in SQL which are used to fetch records from tables and to make relationship among tables or views.
Super Key
Super key is a set of one or more than one keys that can be used to identify a record uniquely in a table.Example : Primary key, Unique key, Alternate key are subset of Super Keys.
Candidate Key
A Candidate Key is a set of one or more fields/columns that can identify a record uniquely in a table. There can be multiple Candidate Keys in one table. Each Candidate Key can work as Primary Key.
Primary Key
Primary key is a set of one or more fields/columns of a table that uniquely identify a record in database table. It can not accept null, duplicate values. Only one Candidate Key can be Primary Key.
Alternate key
A Alternate key is a key that can be work as a primary key. Basically it is a candidate key that currently is not primary key.
Composite/Compound Key
Composite Key is a combination of more than one fields/columns of a table. It can be a Candidate key, Primary key.
Unique Key
Uniquekey is a set of one or more fields/columns of a table that uniquely identify a record in database table. It is like Primary key but it can accept only one null value and it can not have duplicate values.
Foreign Key
Foreign Key is a field in database table that is Primary key in another table. It can accept multiple null, duplicate values.
Primary Key – A primary is a column or set of columns in a table that uniquely identifies tuples (rows) in that table.
Super Key – A super key is a set of one of more columns (attributes) to uniquely identify rows in a table.
Candidate Key – A super key with no redundant attribute is known as candidate key
Alternate Key – Out of all candidate keys, only one gets selected as primary key, remaining keys are known as alternate or secondary keys.
Composite Key – A key that consists of more than one attribute to uniquely identify rows (also known as records & tuples) in a table is called composite key.
Foreign Key – Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables.