在多个表中查找计数

Question

I need to find the amount of times a customer appears in the Orders and Requests tables respectively. However this script is producing the same count value for both places where COUNT is used. The value cannot possibly be the same so what am I doing wrong?

  SELECT   o.CustomerID, 
         COUNT(o.CustomerID) as OrdersPerCustomer,
         COUNT(r.CustomerID) as RequestsPerCustomer
  FROM Orders o
  INNER JOIN  [Customers] c on c.ID = o.CustomerID
  INNER JOIN  [Request] r on r.CustomerID = c.ID
  GROUP BY o.CustomerID

Answer 1

You are multiplying the number of order and request records. Ie by joining the tables, you get for, say, 3 orders and 4 requests for a customer 12 result rows. As the IDs will never be null in a record, COUNT(o.CustomerID) and COUNT(r.CustomerID) are just COUNT(*) (12 in my example, and not 3 and 4 as you expected).

The easiest approach:

select
  customer_id,
  (select count(*) from orders o where o.customerid = c.id) as o_count,
  (select count(*) from request r where r.customerid = c.id) as r_count
from customers c;

The same with subqueries (derived tables) in the from clause:

select
  customer_id,
  coalesce(o.total, 0) as o_count,
  coalesce(r.total, 0) as r_count
from customers c
left join (select customerid, count(*) as total from orders group by customerid) o
  on o.customerid = c.id
left join (select customerid, count(*) as total from request group by customerid) r
  on r.customerid = c.id;

When aggregating from multiple tables, always aggregate first and join then.