How to set two primary keys in mysql

How to set up two primary keys in MySQL

A primary key is a column or combination of columns that uniquely identifies each row in a table. Generally, a table can only have one primary key, but in some special cases, two primary keys are allowed.

Steps:

1. Use composite primary key:

   Using composite primary key is to set two primary keys recommended method. A composite primary key is composed of two or more columns, and the value of each column must be unique.

   <code class="sql">ALTER TABLE table_name ADD PRIMARY KEY (column1, column2);</code>

   Copy after login

   For example, to set a composite primary key for the customer_id and last_name columns in the customers table:

   <code class="sql">ALTER TABLE customers ADD PRIMARY KEY (customer_id, last_name);</code>

   Copy after login

2. Using multi-column primary keys:

   Multi-column primary keys are similar to composite primary keys, but they are specified using special syntax.

   <code class="sql">ALTER TABLE table_name ADD PRIMARY KEY USING INDEX (index_name);</code>

   Copy after login

   where index_name is the name of an existing unique index.

   For example, suppose the customers table has a unique index named customer_index, which can be used as the primary key:

   <code class="sql">ALTER TABLE customers ADD PRIMARY KEY USING INDEX (customer_index);</code>

   Copy after login

   It should be noted that , multi-column primary keys are not available in some MySQL versions.

Advantages and disadvantages:

Advantages of composite primary keys:

• Guaranteed data Uniqueness
• Allows more efficient search and join operations on tables

Disadvantages of composite primary keys:

• Possibly Multiple columns need to be updated to maintain uniqueness
• Creating and managing a composite primary key is more complex than a single primary key

Advantages of multi-column primary keys:

• Simpler and easier to use than a composite primary key
• Use an existing unique index

##Disadvantages of multi-column primary keys:

Not available in some MySQL versions

The above is the detailed content of How to set two primary keys in mysql. For more information, please follow other related articles on the PHP Chinese website!