Which Data Definition Language (DDL) command is used to modify a primary key in an existing table?

The command used to modify a primary key in an existing table is indeed the ALTER TABLE command. This command allows changes to be made to the structure of a table after it has already been created. When modifying a primary key, ALTER TABLE provides the capability to drop the existing primary key constraint or to add a new one.

For instance, if a table requires a different primary key or needs modifications to its constraints, you would utilize ALTER TABLE along with specific sub-commands such as DROP CONSTRAINT to remove the current primary key and ADD CONSTRAINT to define a new primary key. This flexibility in modifying table structures is essential for maintaining database integrity and adapting to changing requirements without having to recreate the table from scratch.

The other commands serve different purposes: CREATE TABLE is used for creating new tables, INSERT is for adding new rows of data, and UPDATE modifies existing data within the table's rows, none of which are relevant for changing the structure pertaining to primary key constraints.