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 after the ALTER TABLE clause.
2. Then, you specify the type of change that you want to make. There are several types of changes that you can make to the structure of a table using the ALTER TABLE statement:
   • ADD: This is used to add a new column into a table.
   • MODIFY: This is used to change the data type of a column.
   • DROP COLUMN: This is used to delete a column in a table.
   • ALTER COLUMN: This is used to change the data type of a column or to change the column size.
   • ADD CONSTRAINT: This is used to add a new constraint to a table.

Here is an example of how to use the ALTER TABLE command:

ALTER TABLE table_name
MODIFY column_name column_type;

In this example, table_name is the name of the table that you want to modify, column_name is the name of the column that you want to change, and column_type is the new data type that you want to set for the column.