SQL左連接外鍵兩個表

Question

我有一個查詢，我正在加入他們問題答案的表格。 兩個表都有一個外鍵，該外鍵鏈接回用戶表以確定誰詢問了誰以及誰回答了問題。

我的問題是，如何修改以下查詢以生成product_question 。 userID和product_answer 。 userID顯示User表中的用戶名，而不僅僅是ID？

                    SELECT `project_question`.`id` AS question_id, `project_question`.`question`, `project_question`.`userID` AS askedBy, 
                           `project_question`.`created` AS question_created, `project_answer`.`id` AS answer_id, 
                           `project_answer`.`answer`, `project_answer`.`userID` AS answeredBy, 
                           `project_answer`.`accepted`, `project_answer`.`created` AS answer_created
                      FROM `project_question`
                 LEFT JOIN `project_answer`
                        ON `project_question`.`id` = `project_answer`.`questionID`
                     WHERE `project_question`.`projectID` = $args[0]
                       AND `project_question`.`projectPhase` = 2

Answer 1

您可以對同一個表和別名使用兩個連接來區分它們：

      SELECT `project_question`.`id` AS question_id, 
             `project_question`.`question`, 
             q_user.`userName` AS askedBy, 
             `project_question`.`created` AS question_created, 
             `project_answer`.`id` AS answer_id, 
             `project_answer`.`answer`, 
             a_user.`userName` AS answeredBy, 
             `project_answer`.`accepted`, 
             `project_answer`.`created` AS answer_created
        FROM `project_question`
   LEFT JOIN `project_answer`
          ON `project_question`.`id` = `project_answer`.`questionID`
  INNER JOIN `User` AS q_user
          ON `project_question`.`userID` = q_user.`userID`
  INNER JOIN `User` AS a_user
          ON `project_answer`.`userID` = a_user.`userID`
       WHERE `project_question`.`projectID` = $args[0]
         AND `project_question`.`projectPhase` = 2