PostgreSQL: combining the number of other tables

Question

I have three tables: users, itemsand boards.

create table users (
    id serial primary key,
    name text
);  

create table items (
    id serial primary key,
    user_id int,
    title text
);  

create table boards (
    id serial primary key,
    user_id int,
    title text
);n

I want to select all users with the number of elements and boards that each user has:

select
    users.*,
    count(items) as item_count,
    count(boards) as board_count
from users
    left join items on items.user_id = users.id
    left join boards on boards.user_id = users.id
group by users.id;

Unfortunately, this does not give me the correct calculations. The calculations are higher than they should be. What is happening, why and how can I fix it?

Answer 1

The problem is that the two left joins create duplicate values.

To solve this problem, you can only read individual values

SELECT u.id, u.name,
       COUNT(DISTINCT i.id) item_count,
       COUNT(DISTINCT b.id) board_count
  FROM users u LEFT JOIN items i
    ON u.id = i.user_id LEFT JOIN boards b
    ON u.id = b.user_id
 GROUP BY u.id, u.name

or count elements and boards per user separately, and then attach them to users

SELECT u.*, 
       COALESCE(i.item_count, 0) item_count, 
       COALESCE(b.board_count, 0) board_count
  FROM users u LEFT JOIN
(
  SELECT user_id, COUNT(*) item_count
    FROM items
   GROUP BY user_id
) i ON u.id = i.user_id LEFT JOIN
(
  SELECT user_id, COUNT(*) board_count
    FROM boards
   GROUP BY user_id
) b ON u.id = b.user_id

SQLFiddle demo

Answer 2

,

select
    u.*,
    (select count(*) from items where user_id = u.id) as item_count,
    (select count(*) from boards where user_id = u.id) as board_count
from users as u
group by u.id;