Which command is used to modify an existing table structure in SQL?

The command used to modify an existing table structure in SQL is indeed the one that begins with "ALTER TABLE." This command allows database administrators and developers to change various aspects of a table's structure after it has been created. For example, it can be used to add new columns, modify existing column data types, rename columns, or even drop columns from the table.

Using "ALTER TABLE" is essential when there is a need to adapt the table to new requirements or changes in the data model without having to recreate the table from scratch. This command is integral for maintaining the integrity and structure of the data as application needs evolve.

For clarity on the other commands: "INSERT INTO" is used to add new rows of data to a table, "UPDATE" is employed to change existing data within the rows, and "DROP TABLE" completely removes a table and its data from the database. None of these commands alter the structural attributes of a table, which is why they are not suitable for modifying a table's structure.