我试图连接这3个表，保留所有值（甚至是null）。我该怎么做却没有du头？
hotel (hid, name, town)
room (hid, num, type)
booking (hid, cid, dstart, ndays, room)
room.hid是hotel.hid的外键。
booking (hid, room)是room (hid, num)的外键。
这是hotel表：
hid name town
-----------------------
H001 Hamlets London
H002 Baileys London
H003 Stevens Kent
H004 Hamlets Kent
这是room表：
hid num type
------------------
H001 1 Double
H001 2 Single
H002 1 Double
H003 1 Single
这是booking表：
hid cid dstart ndays room
----------------------------------------
H001 C001 2019-07-18 5 1
H001 C001 2019-06-20 3 2
H001 C002 2018-06-01 5 1
使用以下查询，我已经将booking表与hotel表结合在一起：
SELECT * FROM hotel h FULL JOIN booking b ON h.hid = b.hid
以下是该查询的结果：
hid name town hid cid dstart ndays room
----------------------------------------------------------------
H001 Hamlets London H001 C001 2019-07-18 5 1
H001 Hamlets London H001 C001 2019-06-20 3 2
H001 Hamlets London H001 C002 2018-06-01 5 1
H003 Stevens Kent null null null null null
H002 Baileys London null null null null null
H004 Hamlets Kent null null null null null
我如何也加入room表，但保持null值？
答案 0 :(得分：2)
要引入room表，您只需在查询中添加一个LEFT JOIN。这不会过滤掉没有预订信息的记录：
SELECT *
FROM hotel h
FULL JOIN booking b ON h.hid = b.hid
LEFT JOIN room r ON r.hid = b.hid AND r.num = b.room
| hid | name | town | hid | cid | dstart | ndays | room | hid | num | type |
| ---- | ------- | ------ | ---- | ---- | ------------------------ | ----- | ---- | ---- | --- | ------ |
| H001 | Hamlets | London | H001 | C001 | 2019-07-18T00:00:00.000Z | 5 | 1 | H001 | 1 | Double |
| H001 | Hamlets | London | H001 | C002 | 2018-06-01T00:00:00.000Z | 5 | 1 | H001 | 1 | Double |
| H001 | Hamlets | London | H001 | C001 | 2019-06-20T00:00:00.000Z | 3 | 2 | H001 | 2 | Single |
| H002 | Baileys | London | | | | | | | | |
| H003 | Stevens | Kent | | | | | | | | |
| H004 | Hamlets | Kent | | | | | | | | |