1. explain the difference between an order by clause and a group by clause. 2. what is the difference between the count aggregate function and the sum aggregate function? 3. in a select query, what is the difference between a where clause and a having clause? 4. why does the order of the operands (tables) matter in an except (minus) query but not in a union query? 5. the relational set operators union, intersect, and except (minus) work properly only when the relations are union-compatible. what does union-compatible mean, and how would you check for this condition?