Define all Key in DBMS From 124study.blogspot.in

A foreign key is a column or columns of data in one table that connects to the primary keydata in the original table.

To ensure the links between foreign key and primary key tables aren't broken, foreign key constraints can be created to prevent actions that would damage the links between tables and prevent erroneous data from being added to the foreign key column.

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Unique key in DBMS Definition from 124study.blogspot.in  November 03, 2018

A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. ... The unique key and primary keyboth provide a guarantee for uniqueness for a column or a set of columns. There is an automatically defined unique key constraint within a primary key constraint.

Definition of Super Key ? From 124Study.blogspot.in November 03, 2018

Definition: A super key is a set or one of more columns (attributes) to uniquely identify rows in a table. Often people get confused between super key and candidate key, so we will also discuss a little about candidate key here.

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Definition of Candidate key . from 124study.blogspot.in  November 03, 2018

A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is unique, and it is called the primarykey.

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Definition of Primary Key in DBMS / From 

124study.blogspot.comNovember 03, 2018

A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records. A primary key'smain features are: It must contain a unique value for each row of data. It cannot contain null values.