mysql
  1. mysql-primary-key
PreviousNext

Primary Key (MySQL Key)

In MySQL, a primary key is a column or combination of columns that uniquely identifies each row in a table. In this tutorial, we'll discuss the syntax for creating primary keys in MySQL, how to use them, and some important points to keep in mind.

Syntax

To create a primary key in MySQL, you can use the following syntax:

ALTER TABLE table_name ADD PRIMARY KEY (column1, column2, ...);

In this syntax, "table_name" is the name of your table, and "column1, column2, ..." are the names of the columns that you want to include in the primary key. You can have one or more columns in a primary key.

You can also create a primary key when creating a new table:

CREATE TABLE table_name (
   column1 datatype,
   column2 datatype,
   ...
   PRIMARY KEY (column1, column2, ...)
);

Example

Let's say we want to create a primary key for a table called "users" that includes the columns "id" and "username". Here's how we can implement it:

ALTER TABLE users ADD PRIMARY KEY (id, username);

Now, the combination of "id" and "username" columns will uniquely identify each row in the table.

Output

There is no specific output when you create a primary key in MySQL. Instead, the primary key constraints are silently added to the table.

Explanation

A primary key is a column or combination of columns that uniquely identifies each row in a table. When you define a primary key constraint on a table, the database engine ensures that each row has a unique combination of values in the primary key columns.

Use

Primary keys are useful for ensuring data integrity and improving the performance of your queries. They provide a way to uniquely identify each row in a table and help prevent duplicate data from being entered into the table.

You can use primary keys to join tables together, and they can also speed up queries that search or group by the primary key columns.

Important Points

  • Every table should have a primary key.
  • Primary keys must be unique and not null.
  • Primary keys can be made up of one or more columns.
  • Primary keys are used to enforce referential integrity and prevent duplicate data.
  • You can use the "SHOW CREATE TABLE" command to view the primary key definition for a table.

Summary

In this tutorial, we discussed how to create primary keys in MySQL using ALTER and CREATE TABLE statements. We also talked about the use cases and benefits of using primary keys, as well as some important points to keep in mind when working with them. With this knowledge, you can now use primary keys to ensure data integrity and improve the performance of your queries in MySQL.

Unique KeyForeign Key
Published on: