How can a primary key be added to a table?

A primary key can indeed be added to a table using the ALTER TABLE command at a later time. This command allows database administrators and developers to modify an existing table structure without needing to recreate the table from scratch. By using the ALTER TABLE command, you can specify that a certain column (or a combination of columns) should be designated as the primary key, which enforces uniqueness and ensures that no two rows in the table can have the same value for that key.

The primary key is crucial for maintaining data integrity and enabling efficient data retrieval. The capability to make changes to an existing table gives flexibility in database management, allowing for the addition of constraints such as primary keys as the design and requirements of the database evolve over time.

In contrast, while a primary key can be defined when a table is created, there are scenarios where it's necessary to add or modify this constraint later, which is why using the ALTER TABLE command is the appropriate choice here.