Housekeeping in R Language you need to do before closing your session. The objects you do not need can delete using housekeeping commands.
SQL Order of execution is sequence of steps. The database executes each Query in a particular order. Read more.
Data access is much faster in Hashing compared to the index. Read the differences between Hashing and Index.
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.
Having Vs Where clause in SQL real usage is to filter rows on aggregate totals. Read more.