COUNT()

How many accounts do we have in music.Account?

SELECT 
	COUNT(*)
FROM music.Account;

How many countries do our users come from?

SELECT
	COUNT(DISTINCT Country)
FROM	music.Users;

SUM()

What’s the total listening time of all the songs in our database?

SELECT
	SUM(duration)
FROM music.Song;

Recall the distinction between SELECT * and SELECT 1

In the former, we return values from every column in the table. In the latter, we map 1 to every tuple in the result set.

Furthermore, when we map 1 and reduce using either SUM(1) or COUNT(1), we find the two expressions are equivalent.

In fact, SUM(1), COUNT(1), and COUNT(*) will return equivalent results - i.e. the number of tuples in each projection.