SQL Order of execution is sequence of steps. The database executes each Query in a particular order. Read more.
SQL ALTER statement you can use to add columns to existing TABLE in RDBMS. Not only TABLE you can read all database object where you can use ALTER.
Ordering the SQL result is possible by giving DESC or ASC in your query. These must be used along with ORDER BY clause. Read more.
SQL Queries you can run and test in your project to update or modify existing rows in a Table.
These are best practices and guide lines to review any SQL program. The example given here is for COBOL.
Index and Key both are different in RDBMS. Take a look all these differences quickly