php - MySql查询2条记录为1条结果

Question

这个问题已经有答案了:

How do you join on the same table, twice, in mysql? (3 个回答)

已关闭 5 年前。

我有一个表地址和一个表路线。

路线存储 2 个地址 ID 以及这些位置之间的距离 -> idStart , idEnd , distance .

+-------+-------+-----+--------+
|idRoute|idStart|idEnd|distance|
+-------+-------+-----+--------+
|0      |1      |3    |2698    |
|1      |2      |4    |914     |
+-------+-------|-----+--------+


+---------+---------+-------------+
|idAddress|country  |city         |
+---------+---------+-------------+
|0        |USA      |Indianapolis |
|1        |Brasil   |Rio          |
|2        |Germany  |Munich       |
|3        |Argentina|Buenos Aires |
|4        |Italy    |Rome         |
+---------+---------+-------------+

`

所以我想查询并获取所有路线，如下所示:

+-------+-------+-------+--------+---------+------+------------+--------+
|idRoute|idStart|idEnd  |CountryS|CountryE |CityS |CityE       |distance|
+-------+-------+-------+--------+---------+------+------------+--------+
|0      |1      |3      |Brasil  |Argentina|Rio   |Buenos Aires|2698    |
|1      |2      |4      |Germany |Italy    |Munich|Rome        |914    |
+-------+-------+-------+--------+---------+------+------------+--------+

如果我执行一些子查询并外部查询特定的路由 id，它会起作用，因为我得到了特定的 where id=#

       SELECT
            ro.idRoute,
            ro.idStart,
            ro.idEnd,
            ro.distance,
            ro.time,
            (select a.country from address a 
            left join route ro 
                on (a.idAddress=ro.idStart)
            WHERE ro.idRoute='.$id .') as country_start,
            (select a.country from address a 
            left join route ro 
                on (a.idAddress=ro.idEnd)
            WHERE ro.idRoute='.$id .') as country_end, FROM route ro
        LEFT JOIN address a ON (a.idAddress=ro.idStart)
        LEFT JOIN countries c ON (a.country=c.code)
        WHERE ro.idRoute='.$id . $this -> presetFilter . ';

但在大多数情况下，我需要所有路由记录，但我无法使其正常工作，因为对于子查询，我会错过 where 语句。

类似外部查询 -> "set fieldname as outerResult "并在内部查询的 where 语句中使用outerResult？？？

Answer 1

您只需从路由表中加入两次地址:一次用于 IDstart，一次用于 IdEnd。这是通过使用表别名来完成的，因此您可以引用地址表两次，并且在连接或引用字段时，数据库引擎知道使用/需要对表的哪个引用。

SELECT r.IdRoute
     , r.idstart
     , r.idend
     , Start_Add.country as CountryS
     , End_Add.country CountryE
     , Start_Add.city as CityS
     , End_Add.city as CityE
     , r.distancce
FROM route r
INNER JOIN Address Start_Add
 on r.IDStart= Start_Add.IdAddress
INNER JOIN address End_Add
 on r.IDEnd= End_Add.IdAddress