計數不正在與LEFT JOIN

Question

我有如下兩個表：

首先是Product

，第二個是Documents

我只想文件爲計數Product表的每個記錄但它只給出一行。我用下面的查詢：

SELECT `p`.`Id`, 
     `p`.`Name`, 
     `u`.`FirstName`, 
     `u`.`LastName`, 
     Count(doc.Id) AS NumOfDocuments 
FROM (`Product` AS p) 
LEFT JOIN `Documents` AS doc ON `p`.`Id` = `doc`.`ProductId` 
INNER JOIN `ProductXUsers` AS pu ON `pu`.`ProductId` = `p`.`Id` 
INNER JOIN `Users` AS u ON `u`.`Id` = `pu`.`UserId` 
AND u.UserType='Customer' 
WHERE `p`.`CreatedBy` = '3' 
GROUP BY p.Id 

Answer 1

在查詢中添加GROUP BY，你可以直接刪除括號包圍Product AS p：

SELECT `p`.`Id`, 
     `p`.`Name`, 
     `u`.`FirstName`, 
     `u`.`LastName`, 
     Count(doc.Id) AS NumOfDocuments 
FROM `Product` AS p 
LEFT JOIN `Documents` AS doc ON `p`.`Id` = `doc`.`ProductId` 
INNER JOIN `ProductXUsers` AS pu ON `pu`.`ProductId` = `p`.`Id` 
INNER JOIN `Users` AS u ON `u`.`Id` = `pu`.`UserId` 
AND u.UserType='Customer' 
WHERE `p`.`CreatedBy` = '3' 
GROUP BY p.Id,`p`.`Name`,`u`.`FirstName`,`u`.`LastName` 

Answer 2

你忘了加上GROUP BY條款。

此外，您可以使用IFNULL替換null值。

SELECT `p`.`Id`, 
     `p`.`Name`, 
     `u`.`FirstName`, 
     `u`.`LastName`, 
     IFNULL(Count(doc.Id),0) AS NumOfDocuments 
FROM `Product` AS p 
INNER JOIN `ProductXUsers` AS pu ON `pu`.`ProductId` = `p`.`Id` 
INNER JOIN `Users` AS u ON `u`.`Id` = `pu`.`UserId` AND u.UserType='Customer' 
LEFT JOIN `Documents` AS doc ON `p`.`Id` = `doc`.`ProductId` 
WHERE `p`.`CreatedBy` = '3' 
GROUP BY p.Id