Improve description of statements/clauses (checkout)

[anneglaser]

It would be helpful to add a quick description of some of the statements used. Mainly the select statement since I know when I first started I had concerns on if I was changing the underlying data or not. Also the group by, it would be useful to call out that the group by identifier must exist in the select statement.

Description: SELECT statement can access data in a database and returned specified columns, but it cannot manipulate the data where it lives in the database. However, you can manipulate the data returned to you in your SELECT statement, but it won't save to the database.

Description: GROUP BY selection must exist in the select statement.