如何获得表A所有数据与表B只有与表A匹配的行A

Question

我有两个表我想选择所有记录在cardId 2表中的位置，并且如果一个位置被映射到location_card表中，那么地点的名称

它可以通过单独查询然后在脚本中加入它们，但是我想单个查询。我试着用下面的查询，但这个功能工作。

SELECT `cl`.`locationId`, `cl`.`cardId`, `l`.`name` 
FROM (`locations` as l) 
JOIN `location_cards` as cl ON `l`.`cardId` = `cl`.`cardId` 
WHERE `l`.`id` = '2' 



location_cards 
+---+-------------+-------+ 
|id | locationId |cardId | 
+---+-------------+-------+ 
| 1 | 1   | 1 | 
| 2 | 1   | 2 | 
| 3 | 3   | 2 | 
+---+-------------+-------+ 


locations 
+---+-------------+-------+ 
|id | name  |cardId | 
+---+-------------+-------+ 
| 1 | some  | 1 | 
| 2 | pla   | 2 | 
| 3 | sdsdffsdf | 2 | 
+---+-------------+-------+ 

Answer 1

您需要更改为LEFT JOIN，但它也出现在WHERE子句使用了错误的过滤器：

select c.locationid, 
    c.cardid, 
    l.name 
from locations l 
left join location_cards c 
    on l.cardid = c.cardid 
where l.cardid = 2 

看到SQL Fiddle with Demo

Answer 2

试着改变你的JOIN到LEFT JOIN和秩序的联接。

SELECT `cl`.`locationId`, `cl`.`cardId`, `l`.`name` 
FROM `location_cards` as cl 
left JOIN `locations` as l ON `l`.`cardId` = `cl`.`cardId` 
WHERE `l`.`id` = '2'