Question

我希望在我的SQL中获得一个出发地和目的地郊区。

SELECT jp.`id`,
     CONCAT(u.`first_name`, " ", u.`last_name`) AS `name`,
     jp.`departure_time`,
     jp.`destination_time`,
     s.`suburb` AS `departure_suburb`


FROM `journey_planning` AS `jp`
JOIN `users` AS `u` ON jp.`driver_id` = u.`id`
JOIN `suburbs` AS s ON jp.`departure_suburb_id` = s.`id`
  OR jp.`destination_suburb_id` = s.`id`

如您所见，有一个departure_suburb_id和destination_suburb_id。我需要它们映射到suburbs表中的相对值。

此刻，这将返回两行，除了不同的郊区名称外，两行都有相同的信息（但显然都是departure_suburb。）

我不是SQL专家，所以我不确定我应该做什么。我考虑过使用子查询，但我认为没有它们可能有一种方法可以避免两次查找。

虽然departure_suburb和destination_suburb指向suburbs表中的正确值，但我应该如何修改此查询以返回现在的状态？

Answer 1

您需要两次加入郊区表，一次是出发，一次是目的地。即使隐含“INNER JOIN”，也可以输入您正在使用的联接类型，这也是一个好习惯！

SELECT jp.`id`,
     CONCAT(u.`first_name`, " ", u.`last_name`) AS `name`,
     jp.`departure_time`,
     jp.`destination_time`,
     s.`suburb` AS `departure_suburb`
     s2.`suburb` AS `destination_suburb` 

FROM `journey_planning` AS `jp`
JOIN `users` AS `u` ON jp.`driver_id` = u.`id`
JOIN `suburbs` AS s ON jp.`departure_suburb_id` = s.`id`
JOIN `suburbs` AS s2 ON jp.`destination_suburb_id` = s2.`id`

Answer 2

SELECT
    jp.id
  , CONCAT(u.first_name, " ", u.last_name) AS name
  , jp.departure_time
  , jp.destination_time
  , dep.suburb AS departure_suburb
  , dest.suburb AS destination_suburb

FROM journey_planning AS jp
  JOIN users AS u 
    ON jp.driver_id = u.id
  JOIN suburbs AS dep 
    ON jp.departure_suburb_id = dep.id
  JOIN suburbs AS dest 
    OR jp.destination_suburb_id = dest.id

怎么可以这样做呢？（对于像你这样的简单JOIN的查询，我不建议这样做）。由于jp表上的所有关系都是1对多，因此可以这样做：

SELECT
    jp.id
  , CONCAT( ( SELECT first_name
              FROM users
              WHERE jp.driver_id = users.id
            )
          , " "
          , ( SELECT last_name
              FROM users
              WHERE jp.driver_id = users.id
            )
          )
      AS name
  , jp.departure_time
  , jp.destination_time
  , ( SELECT suburb
      FROM suburbs
      WHERE jp.departure_suburb_id = suburbs.id
    ) 
      AS departure_suburb
  , ( SELECT suburb
      FROM suburbs
      WHERE jp.destination_suburb_id = suburbs.id
    ) 
      AS destination_suburb

FROM journey_planning AS jp

正如您所看到的，它变得非常复杂，尤其是当您想要从表中显示多个字段时，例如表用户的名字和姓氏。

Answer 3

使用不同的表别名再次加入郊区：

SELECT jp.`id`,
    CONCAT(u.`first_name`, " ", u.`last_name`) AS `name`,
    jp.`departure_time`,
    jp.`destination_time`,
    departure_suburb.`suburb` AS `departure_suburb`,
    destination_suburb.`suburb` AS `destination_suburb`
FROM `journey_planning` AS `jp`
    JOIN `users` AS `u` ON jp.`driver_id` = u.`id`
    JOIN `suburbs` AS depart_suburb ON jp.`departure_suburb_id` = s.`id`
    JOIN `suburbs` AS destination_suburb ON jp.`destination_suburb_id` = ss.`id`

如何通过SQL获取这两个郊区？

3 个答案: