Tag: Unique Keys

Video tutorial illustrates the use of Primary Key, Foreign Key, Unique Key, AUTO_INCREMENT and NOT NULL in MySQL.

Primary Key
The PRIMARY KEY constraint uniquely identifies each record in a database table.
Primary keys have unique values.
Primary Keys are NOT NULL by default.

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CREATE TABLE Apple
(
 id int NOT NULL,
 name varchar(255) NOT NULL,
 PRIMARY KEY(id)
);

Unique Key
The UNIQUE constraint uniquely identifies each record in a database table.

UNIQUE keys are not Primary Keys. but all Primary keys are Unique keys.

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CREATE TABLE Apple
(
 id int NOT NULL,
 name varchar(255) NOT NULL,
 UNIQUE(id)
);

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

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CREATE TABLE Orders
(
 id int NOT NULL,
 O_No int NOT NULL,
 PRIMARY KEY(O_No),
 FOREIGN KEY(id) REFERENCES Apple(id)
);

If your MySQL uses InnoDB engine, then use the following query:

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CREATE TABLE Orders
(
 id int NOT NULL,
 O_No int NOT NULL,
 PRIMARY KEY(O_No),
 FOREIGN KEY(id) REFERENCES Apple(id)
)ENGINE=InnoDB;

Video Tutorial: Primary Foreign Unique Keys, AUTO_INCREMENT: MySQL

phpMyAdmin UI Interface Options
Options for assigning Primary key, unique key to the attributes..also the edit and delete keys.

AUTO_INCREMENT and NOT NULL

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CREATE TABLE Apple
(
 id int NOT NULL AUTO_INCREMENT,
 name varchar(255) NOT NULL,
 PRIMARY KEY(id)
);

AUTO_INCREMENT is used for integer type data.. the advantage being, we need not explicitly send the data to this field. MySQL automatically inserts the value to this field.