Find the total number of members (including guests) who have made at least one booking.
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select count(distinct memid) from cd.bookings          

Your first instinct may be to go for a subquery here. Something like the below:

select count(*) from 
	(select distinct memid from cd.bookings) as mems

This does work perfectly well, but we can simplify a touch with the help of a little extra knowledge in the form of COUNT DISTINCT. This does what you might expect, counting the distinct values in the passed column.

Take a look at COUNT DISTINCT

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