Question

Produce a list of facilities with a total revenue less than 1000. Produce an output table consisting of facility name and revenue, sorted by revenue. Remember that there's a different cost for guests and members!
Schema reminder
DB schema

Expected Results

name revenue
Table Tennis 180
Snooker Table 240
Pool Table 270

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Answers and Discussion Show

select name, revenue from (
	select facs.name, sum(case 
				when memid = 0 then slots * facs.guestcost
				else slots * membercost
			end) as revenue
		from cd.bookings bks
		inner join cd.facilities facs
			on bks.facid = facs.facid
		group by facs.name
	) as agg where revenue < 1000
order by revenue;          

You may well have tried to use the HAVING keyword we introduced in an earlier exercise, producing something like below:

select facs.name, sum(case 
		when memid = 0 then slots * facs.guestcost
		else slots * membercost
	end) as revenue
	from cd.bookings bks
	inner join cd.facilities facs
		on bks.facid = facs.facid
	group by facs.name
	having revenue < 1000
order by revenue;

Unfortunately, this doesn't work! You'll get an error along the lines of ERROR: column "revenue" does not exist. Postgres, unlike some other RDBMSs like SQL Server and MySQL, doesn't support putting column names in the HAVING clause. This means that for this query to work, you'd have to produce something like below:

select facs.name, sum(case 
		when memid = 0 then slots * facs.guestcost
		else slots * membercost
	end) as revenue
	from cd.bookings bks
	inner join cd.facilities facs
		on bks.facid = facs.facid
	group by facs.name
	having sum(case 
		when memid = 0 then slots * facs.guestcost
		else slots * membercost
	end) < 1000
order by revenue;

Having to repeat significant calculation code like this is messy, so our anointed solution instead just wraps the main query body as a subquery, and selects from it using a WHERE clause. In general, I recommend using HAVING for simple queries, as it increases clarity. Otherwise, this subquery approach is often easier to use.

You may find HAVING difficult to use here. Try a subquery instead. You'll probably also need a CASE statement.

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