如何使用＆＃34;其中＆＃34;在加入两个表？

Question

主表：

（跳闸）

id | name
1  | USA
2  | Europe
3  | Asia

儿童：

(plane)
id | trip_id | name | other
1  |    1    | aaa  |  w
2  |    1    | bbb  |  e
3  |    3    | ccc  |  rr

(boat)
id | trip_id | name
1  |    2    | jjj
2  |    2    | kkk
3  |    3    | lll

如果我想乘飞机旅行，那么我可以：

SELECT trip.* FROM trip INNER JOIN plane ON plane.trip_id = trip.id

如果我想乘船去旅行，那么我可以：

SELECT trip.* FROM trip INNER JOIN boat ON boat.trip_id = trip.id

但如何使用飞机或船只（或两者）进行一次查询？

Answer 1

只需使用UNION ALL：

SELECT trip.* FROM trip INNER JOIN plane ON plane.trip_id = trip.id
UNION ALL
SELECT trip.* FROM trip INNER JOIN boat ON boat.trip_id = trip.id

Answer 2

您可以通过LEFT加入

来实现

SELECT trip.*,ifnull(plane.Name,'NA') as planetrip, ifnull(boat.Name,'NA')  boattrip FROM trip 
LEFT JOIN plane ON plane.trip_id = trip.id
LEFT JOIN boat ON boat.trip_id = trip.id
WHERE (plane.Name IS NOT NULL OR boat.Name IS NOT NULL)

SQL DEMO：http://sqlfiddle.com/#!9/2ae5c/10

Answer 3

尝试将trip表连接到其他两个表。这里的诀窍是我们最后通过旅行进行汇总，并检测每次旅行中是否有飞机或船只表中的匹配。

SELECT t.id, t.name,
    CASE WHEN COUNT(p.trip_id) > 0 THEN 'yes' ELSE 'no' END AS has_plane,
    CASE WHEN COUNT(b.trip_id) > 0 THEN 'yes' ELSE 'no' END AS has_boat
FROM trip t
LEFT JOIN plane p
    ON t.id = p.trip_id
LEFT JOIN boat b
    ON t.id = b.trip_id
GROUP BY t.id;

    id  name    has_plane   has_boat
1   1   USA     yes         no
2   2   Europe  no          yes
3   3   Asia    yes         yes

Demo

Answer 4

您可以使用左连接，如下所示

select trip.*, isnull(plane.name, '-') as PlaneName, isnull(boat.name, '-') as BoatName from trip left join plane ON plane.trip_id = trip.id left JOIN boat ON boat.trip_id = trip.id

它将返回下表，随意使用这3个表中任何一个的任何字段...

Answer 5

如果您想要在多行上获得所有结果，可以使用UNION 但你的桌面和船的列数不同，所以你应该使用带有明确列名的联合，例如：

  SELECT trip_id, plane.name, trip.name
  from plane
  inner join trip on trip.id = plane.trip_id
  UNION 
  SELECT trip_id, plane.name, trip.name
  from bout
  inner join trip on trip.id = boat.trip_id

或者如果您需要所有值，则必须在需要时使用空列

  SELECT trip_id, plane.name, trip.name, other
  from plane
  inner join trip on trip.id = plane.trip_id
  UNION 
  SELECT trip_id,plane.name, trip.name, null
  from bout
  inner join trip on trip.id = boat.trip_id