Which SQL command is used to update the structure of an existing table?

• A. UPDATE TABLE
• B. ALTER TABLE
• C. MODIFY TABLE
• D. CREATE TABLE

Answer: (B)

The correct command to update the structure of an existing table in SQL is ALTER TABLE. This command allows users to make various modifications to a table's structure, such as adding new columns, removing existing columns, or changing the data type of a column.

For instance, if you want to add a new column to a table, you would use ALTER TABLE followed by the table name and the command to add the new column. This command is fundamental for database management, allowing for the dynamic adjustment of a database's schema without needing to recreate the table entirely.

The other options, such as UPDATE TABLE and MODIFY TABLE, are not standard SQL commands. While UPDATE is used to modify data within the existing records of a table, it does not alter the structure of the table itself. CREATE TABLE is specifically used to create a new table in the database rather than modifying an existing one. Therefore, ALTER TABLE is the appropriate choice for changing a table's structure.