How to add primary key constraints in sql

Adding primary key constraints through SQL ensures data integrity and consistency. The steps include: 1. Determine the primary key column, which must be unique and not nullable; 2. Add constraints using the ALTER TABLE table_name ADD PRIMARY KEY (column_name1, column_name2, ...) statement.

How to add primary key constraints using SQL

A primary key constraint is a column or group of columns that uniquely identifies each record in the table. Adding primary key constraints ensures data integrity and consistency in the database.

Steps to add primary key constraints:

1. Determine the primary key column: Select one or more columns as primary keys. The primary key column must have a unique and non-null value.
2. Write SQL statements: Add primary key constraints using the following syntax:

 <code class="sql">ALTER TABLE table_name ADD PRIMARY KEY (column_name1, column_name2, ...);</code>

in:

• table_name is the table name you want to add the primary key constraint.
• column_name1 , column_name2 , ... are the column names that make up the primary key.

Example:

Suppose you have a table named students that contains student_id , name , age columns. To set the student_id column as primary key, use the following SQL statement:

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

Notes:

• Once the primary key constraint is added, the newly inserted data must have a unique primary key value.
• Deleting or updating primary key values ​​can result in violation of primary key constraint errors.
• You can use DROP PRIMARY KEY constraint to delete the primary key constraint from the table.

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