Question

I’m trying to do something like:

SELECT c.id, c.name, COUNT(orders.id)
FROM customers c
JOIN orders o ON o.customerId = c.id

However, SQL will not allow the COUNT function. The error given at execution is that c.Id is not valid in the select list because it isn’t in the group by clause or isn’t aggregated.

I think I know the problem, COUNT just counts all the rows in the orders table. How can I make a count for each customer?

EDIT

Full query, but it’s in dutch… This is what I tried:

select k.ID,
       Naam,
       Voornaam,
       Adres,
       Postcode,
       Gemeente,
       Land,
       Emailadres,
       Telefoonnummer,
       count(*) over (partition by k.id) as 'Aantal bestellingen',
       Kredietbedrag,
       Gebruikersnaam,
       k.LeverAdres,
       k.LeverPostnummer,
       k.LeverGemeente,
       k.LeverLand
from klanten k
  join bestellingen on bestellingen.klantId = k.id

No errors but no results either..

Answer 1

If you really want to be able to select all of the columns in Customers without specifying the names (please read this blog post in full for reasons to avoid this, and easy workarounds), then you can do this lazy shorthand instead:

;WITH o AS 
(
  SELECT CustomerID, CustomerCount = COUNT(*)
  FROM dbo.Orders GROUP BY CustomerID
)
SELECT c.*, o.OrderCount
FROM dbo.Customers AS c
INNER JOIN dbo.Orders AS o
ON c.id = o.CustomerID;

EDIT for your real query

SELECT 
  k.ID, 
  k.Naam, 
  k.Voornaam, 
  k.Adres, 
  k.Postcode, 
  k.Gemeente, 
  k.Land, 
  k.Emailadres, 
  k.Telefoonnummer,
  [Aantal bestellingen] = o.klantCount, 
  k.Kredietbedrag, 
  k.Gebruikersnaam, 
  k.LeverAdres, 
  k.LeverPostnummer, 
  k.LeverGemeente, 
  k.LeverLand
FROM klanten AS k 
INNER JOIN 
(
  SELECT klantId, klanCount = COUNT(*)
  FROM dbo.bestellingen 
  GROUP BY klantId
) AS o
ON k.id = o.klantId;

I think this solution is much cleaner than grouping by all of the columns. Grouping on the orders table first and then joining once to each customer row is likely to be much more efficient than joining first and then grouping.