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DATABASE KEYS CODE DEMO

The sql file has code that can be used to demonstrate a way to apply each of the primary, unique and foreign key constraints to a given table and the issues one can encounter if the data added to the table violates these constraints. The code has embedded insertion queries for some sample data for anyone to be able to execute the SQL on their local.

The example considered to demonstrate this topic is of an owners and pets table . The owners table stores a list of owners of pets that have attributes such as owner id, name, city , email id and mobile number. The pets table stores a list of pets that the owners have with attributes such as pet id , name , type , owner id .

Below are the columns in both the tables that have constraints defined.

Owners table:

owner id - Primar key email id - unique key mobile number- unique key

Pets table:

petid - Primary key owner id - foreign keey

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Keys:

Primary Key

  1. A column/Attribute that can uniquely identify each row.
  2. The value in this column has to unique for each row and non null.
  3. A table is allowed to have only one primary key.
  4. Primary key is declared by applying the 'Primary Key' constraint in the table structure.

Unique Key

  1. The value in this column has to be unique for each row.
  2. But it differs from Primary key that values can be null in this column.
  3. A table can have more than one unique keys (more than one columns can have the unique key constraint).
  4. Unique key is declared by applying the 'unique' constraint in the table structure.

Foreign Key

  1. Foreign Key is used to link 2 tables - It is a field in the table that is Primary key in another table.
  2. The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
  3. The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table.
  4. It is declared by using the 'foreign key references' in the table structure of the child tables