Which command is used to modify the table structure such as key constraints and Column size?

The command used to modify the table structure such as key constraints and column size in SQL (Structured Query Language) is ALTER TABLE.

Here is a step by step guide on how to use it:

1. First, you need to specify the table name that you want to change. You do this by using the ALTER TABLE command followed by the name of the table. For example: ALTER TABLE table_name.

2. Next, you specify what kind of modification you want to make. There are several options here, such as ADD, DROP, MODIFY, etc.

   • If you want to add a new column, you would use the ADD command followed by the column name and the data type. For example: ALTER TABLE table_name ADD column_name data_type.

   • If you want to delete a column, you would use the DROP COLUMN command followed by the column name. For example: ALTER TABLE table_name DROP COLUMN column_name.

   • If you want to change the data type of a column, you would use the MODIFY command followed by the column name and the new data type. For example: ALTER TABLE table_name MODIFY column_name new_data_type.

3. Finally, you would run the command. If there are no errors in your SQL syntax, the table structure will be modified according to your specifications.