如何在mysql查询中获取每个记录的计数

Question

I have two mysql tables, users and orders to track orders placed by each user.

I want to get a list of each user, and the total number of orders.

For an individual, I could use:

SELECT users.name, (select count(*) from orders where orders.users_id = 1) as total
from users
where users.id = 1

Is there an efficient way to do this for all users simultaneously?

Answer 1

You can do it with a correlated subquery.

SELECT users.name, (select count(*) from orders where orders.users_id = users.id) as total
from users

But a better way is with a join.

SELECT users.name, IFNULL(COUNT(orders.id), 0) AS total
FROM users
LEFT JOIN orders ON orders.users_id = users.id
GROUP BY users.id

It may actually be better to join with a subquery that aggregates, rather than aggregating after joining:

SELECT u.name, IFNULL(o.total, 0) AS total
FROM users as u
LEFT JOIN (
    SELECT users_id, COUNT(*) AS total
    FROM orders
    GROUP BY users_id) AS o ON u.id = o.users_id

Answer 2

The key is really a GROUP BY:

SELECT u.id, u.name, COUNT(*)
FROM users u
INNER JOIN orders o ON u.id = o.users_id
GROUP BY u.id, u.name

Answer 3

You could do something like that too, in this case only will return when there are records on order :

   SELECT users.name, count(*) 
     from users
     join orders On orders.users_id = users.id  

    group by users.name