Question

I am looking at the following materialized view

CREATE MATERIALIZED VIEW ShipStats AS
SELECT country, AVG(displacement), COUNT(*)
FROM classes, ships
WHERE classes.class = ships.name
GROUP BY country;

and trying to figure out what the SELECT [] part of it is doing.

I understand select country. and AVG(displacement) is the average of the displacement column. But what is COUNT(*) doing. Also what would the AVG displacement have to do with a statistic for a single ship?

Relations:

Classes(class, type, country, numGuns, bore, displacement)
Ships(name, class, launched)

Answer 1

count(*) simply returns the amount of rows per group.

Because of the group by statement all rows of the same country are merged into a single row. count(*) return the amount of rows that go into one such group, while AVG(displacement) is the average value of displacement over all rows in that group.

So the displacement is not for a single ship, but for a group of ships.

If there would be just one row going into such a group, count would return 1 and average would return the displacement value of that row.