What are primary keys and foreign keys

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  Vithal Wadje13y agoPosted Nov 19, 2012, 5:00 AM

  The PRIMARY KEY constraint uniquely identifies each record in a database table.

  Primary keys must contain unique values.

  A primary key column cannot contain NULL values.

  Each table should have a primary key, and each table can have only ONE primary key

  
  A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

  The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

  The FOREIGN KEY constraint also prevents that invalid data form being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.
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  Vishnujeet Kumar14y agoPosted Nov 9, 2012, 12:08 PM

  Rahul,
  very good video tutorial.
  
  https://www.youtube.com/watch?v=8iIvyNPwruU
  
  https://www.youtube.com/user/dnfvideo/videos?view=0
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  Satyapriya Nayak14y agoPosted Nov 9, 2012, 10:55 AM

  http://www.1keydata.com/sql/sql-primary-key.html
  
  http://www.1keydata.com/sql/sql-foreign-key.html
  
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  Vulpes14y agoPosted Nov 9, 2012, 9:20 AM

  A primary key is used to uniquely identify each record in a database table.

  
  

  Consequently, primary keys must contain unique values and cannot be null.

  
  

  However, a primary key needn't consist of a single column but can be formed from a combination of several columns.

  
  

  Foreign keys are used to establish links between tables.

  
  

  A foreign key in one table refers to a primary key in another table.