COUNT Example Setup

tables.sql

DROP TABLE IF EXISTS users;
CREATE TABLE users (user_id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(20));
INSERT INTO users (name) VALUES ('Matt');
INSERT INTO users (name) VALUES ('Simon');
INSERT INTO users (name) VALUES ('Jen');

DROP TABLE IF EXISTS posts;
CREATE TABLE posts (post_id INT PRIMARY KEY AUTO_INCREMENT, user_id INT);
INSERT INTO posts (user_id) VALUES (1);
INSERT INTO posts (user_id) VALUES (1);
INSERT INTO posts (user_id) VALUES (1);
INSERT INTO posts (user_id) VALUES (2);
INSERT INTO posts (user_id) VALUES (2);

DROP TABLE IF EXISTS pages;
CREATE TABLE pages (page_id INT PRIMARY KEY AUTO_INCREMENT, user_id INT);
INSERT INTO pages (user_id) VALUES (2);
INSERT INTO pages (user_id) VALUES (2);
INSERT INTO pages (user_id) VALUES (3);
INSERT INTO pages (user_id) VALUES (3);
INSERT INTO pages (user_id) VALUES (3);
INSERT INTO pages (user_id) VALUES (3);
INSERT INTO pages (user_id) VALUES (3);

+---------+-------+
| user_id | name  |
+---------+-------+
|       1 | Matt  |
|       2 | Simon |
|       3 | Jen   |
+---------+-------+

+---------+---------+
| post_id | user_id |
+---------+---------+
|       1 |       1 |
|       2 |       1 |
|       3 |       1 |
|       4 |       2 |
|       5 |       2 |
+---------+---------+

+---------+---------+
| page_id | user_id |
+---------+---------+
|       1 |       2 |
|       2 |       2 |
|       3 |       3 |
|       4 |       3 |
|       5 |       3 |
|       6 |       3 |
|       7 |       3 |
+---------+---------+