SQL - Using COUNT with alias shorthand - sql

Question

I'm learning SQL and having trouble performing a query that uses COUNT.
I have a movies database with three tables:
Table public.movies:
Column | Type | Collation | Nullable | Default
--------------+---------+-----------+----------+------------------------------------
id | integer | | not null | nextval('movies_id_seq'::regclass)
title | text | | not null |
release_year | integer | | not null |
runtime | integer | | not null |
rating | text | | not null |
studio_id | integer | | |
Table public.stars:
Column | Type | Collation | Nullable | Default
------------+---------+-----------+----------+-----------------------------------
id | integer | | not null | nextval('stars_id_seq'::regclass)
first_name | text | | not null |
last_name | text | | |
birth_date | date | | not null |
Indexes:
"stars_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "roles" CONSTRAINT "roles_star_id_fkey" FOREIGN KEY (star_id) REFERENCES stars(id) ON DELETE CASCADE
Table public.roles:
Column | Type | Collation | Nullable | Default
----------+---------+-----------+----------+-----------------------------------
id | integer | | not null | nextval('roles_id_seq'::regclass)
movie_id | integer | | |
star_id | integer | | |
Indexes:
"roles_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
"roles_movie_id_fkey" FOREIGN KEY (movie_id) REFERENCES movies(id) ON DELETE CASCADE
"roles_star_id_fkey" FOREIGN KEY (star_id) REFERENCES stars(id) ON DELETE CASCADE
I'm trying to select the first and last names of every star along with the number of movies they have been in, and group everything by first and last name (as to not have any duplicate stars). This is what I tried:
SELECT s.first_name, s.last_name, m.COUNT(*)
FROM movies m
JOIN roles r
ON m.id = r.movie_id
JOIN stars s
ON r.star_id = s.id
GROUP BY s.first_name, s.last_name;
I keep getting ERROR: schema "m" does not exist.
I'd appreciate any pointers.

Answer 1

The correct syntax just uses COUNT(*):
SELECT s.first_name, s.last_name, COUNT(*)
FROM movies m JOIN
roles r
ON m.id = r.movie_id JOIN
stars s
ON r.star_id = s.id
GROUP BY s.id, s.first_name, s.last_name;
To avoid duplicates, use the primary key in the GROUP BY.
Note that you don't need movies in the query:
SELECT s.first_name, s.last_name, COUNT(*)
FROM roles r JOIN
stars s
ON r.star_id = s.id
GROUP BY s.id, s.first_name, s.last_name;
Finally, if a star can play more than one role in a movie, you will want to use COUNT(DISTINCT r.movie_id) in the SELECT.