How to add primary key column in SQL

To add a primary key column to a table, you can use the SQL statement: ALTER TABLE table_name ADD PRIMARY KEY (column_name), where table_name is the table name and column_name is the column name specified as the primary key. The steps are as follows: 1. Connect to the database; 2. Execute the ALTER TABLE statement to add the primary key column. The primary key column ensures that there are no duplicate values ​​in the table, optimizes query speed, and is used to create reference integrity constraints.

How to add primary key columns using SQL

Introduction

The primary key column is the key value in the database table that uniquely identifies each row. Adding primary key columns to an existing table is a common task and this article will explain in detail how to do this using SQL statements.

grammar

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

parameter

• table_name: The table name to add the primary key column.
• column_name: The column name to be used as the primary key.

step

1. Connect to the database: Use the SQL client or command line tool to connect to the database you want to modify.
2. Use the ALTER TABLE statement: Enter the ALTER TABLE statement followed by the ADD PRIMARY KEY clause and the column name.
3. Execute statement: Execute statement to add primary key column to the table.

Example

Consider a table named "Customers" with the "Customer_ID" column. To specify this column as a primary key, execute the following statement:

 <code class="sql">ALTER TABLE Customers ADD PRIMARY KEY (Customer_ID);</code>

advantage

Advantages of adding primary key columns include:

• Data Integrity: The primary key ensures that there are no duplicate values ​​in the table.
• Query speed: Primary key columns are used to optimize queries so that the database can find data faster.
• Reference Integrity: The primary key column is used to create reference integrity constraints for other tables and the table.

Things to note

• Make sure the selected primary key column is unique and not null.
• You can only add primary keys to tables that do not have existing primary keys.
• If the column contains existing data, you must make sure that the data is unique. Otherwise, adding the primary key will result in an error.

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