What is a join command? In addition, How can you combine two tables/views together? For instance one table contains 100 rows and the other one contains 200 rows, have exactly the same fields and you want to show a query with all data (300 rows).

And

Why can a "group by" or "order by" clause be expensive to process? What is the difference between a where and having clause?

I need another explaination other than whats on here already please..