如何使用計數函數連接三個表以返回零結果？

Question

我有這個查詢，有三個表（發票，customer_addresses，國家），我想返回每個國家的銷售數量，有些國家沒有銷售，所以我想返回零作爲sales_count以外的其他排除它們從結果。

SELECT c.name,c.iso_code,c.id as country_id, COUNT(*) as sales_count ,SUM(a.total_due) as gross 
     FROM `invoices` a 
      JOIN `customer_addresses` b 
      ON b.`customer_id` = a.`customer_id` 
      JOIN `countries` c 
      ON c.`id` = b.`country_id` 
     WHERE a.`status` = 'Paid' AND a.`deleted` ='No' 
     GROUP BY c.name 

樣本數據：

發票

id | total_due | customer_id 
1 | 25  |  5 
2 | 45  |  7 

customer_addresses

id | address | customer_id | country_id 
5 | some address | 1   | 3 
7 | some address | 2   | 4 

個國家

id |  name  | iso_code 
3 | USA   | US 
4 | United Kingdom | UK 
5 | France   | Fr 

Answer 1

你可以使用一個right join代替(inner) join，但它已不常用，它通常被認爲是可讀的甚少。

因此，您可以使用left join作爲表格，並使country成爲第一個表格。使用left join，即使連接表中沒有匹配的行，仍然會得到結果。撥打count的電話也稍有改動。 count(*)將只返回行數，因此無回票的國家將返回1。 count(invoice_id)只計算那些行數invoice_id is not null，因此對於沒有銷售額的國家/地區將返回0。

SELECT 
    c.name, c.iso_code, c.id as country_id, 
    COUNT(a.invoice_id) as sales_count, 
    SUM(a.total_due) as gross 
FROM 
    `countries` c 
    LEFT JOIN `customer_addresses` b 
    ON c.`id` = b.`country_id` 
    LEFT JOIN `invoices` a 
    ON b.`customer_id` = a.`customer_id` 
    AND a.`status` = 'Paid' 
    AND a.`deleted` ='No' 
GROUP BY c.name