MySQL的左连接不止一次

Question

我有这样的一个表：

数据：

id | state | more 
----------------- 
1 | 1  | ... 
2 | 2  | ... 
3 | 1  | ... 

，并指出：

id | state 
---------- 
1 | Open 
2 | Closed 

如果我做

SELECT states.state FROM data LEFT JOIN states ON data.state = states.id 

我收到

state 
----- 
Open 
Closed 
Open 

到目前为止这么好。我所试图做的（没有成功），如果我有这样的一个表中的数据：

id | state | state2 | more 
-------------------------- 
1 | 1  | 2  | ... 
2 | 2  | 1  | ... 
3 | 1  | 1  | ... 

如何获得：

state | state 2 
---------------- 
Open | Closed 
Closed | Open 
Open | Open 

Answer 1

SELECT states.state, states2.state 
FROM data 
LEFT JOIN states as states ON data.state = states.id 
LEFT JOIN states as states2 ON data.state2 = states2.id 

Answer 2

您需要做的另一个左连接

SELECT 
    s1.state AS state1, 
    s2.state AS state2 
FROM data d 
LEFT JOIN states s1 ON d.state = s1.id 
LEFT JOIN states s2 ON d.state2 = s2.id 

Answer 3

只是用另一LEFT JOIN和alias

SELECT 
    s1.state AS state1 
    s2.state AS state2 
FROM data d 
LEFT JOIN states s1 ON d.state = s1.id 
LEFT JOIN states s2 ON d.state2 = s2.id