如何在SQL查詢中組合來自4個不同表的信息？

Question

我有一個SQL查詢來列出比一個訂單製作更多 的客戶的姓名和總支出。以下列出了所有相關信息，但我正在努力研究如何向前邁進。

SELECT l.quantity, o.orderID, i.itemID, o.custID, i.unitcost, c.familyname 
FROM lineitems l, orders o, items i, customers c 
WHERE l.itemID = i.itemID 
AND c.custID = o.custID 
AND o.orderID = l.orderID 
ORDER BY o.custID 

Answer 1

Select customerId, Sum(i.quantity*i.unitCost) 
From lineitems I 
    join orders o on o.orderID = i.orderID 
where Exists(Select * From orders 
      where customerId = o.customerId 
      having count(*) > 1) 
group by customerId 

，或者用名稱，而不只是客戶ID

Select c.familyname, Sum(i.quantity*i.unitCost) 
From lineitems I 
    join orders o on o.orderID = i.orderID 
    join customers c on c.customerId = o.customerId 
where Exists(Select * From orders 
      where customerId = o.customerId 
      having count(*) > 1) 
group by c.familyname