从MySQL中的同一表联接两个字段

Question

SELECT  pms.id, 
                    pms.id_from, 
                    pms.id_to, 
                    pms.fecha, 
                    pms.leida, 
                    usuarios.img_src  as avatar_from, 
                    usuarios.alias as name_from,
                    usuarios.img_src  as avatar_to, 
                    usuarios.alias as name_to  
FROM pms 
LEFT JOIN usuarios ON usuarios.id = pms.id_from
LEFT JOIN usuarios ON usuarios.id = pms.id_to

每行有两个引用用法（别名或img_src）的问题：

我得到这个错误：

The following errors were reported:
Not unique table/alias: 'usuarios'

或这个

SELECT  pms.id, 
                pms.id_from, 
                pms.id_to, 
                pms.fecha, 
                pms.leida, 
                usuarios.img_src  as avatar_from, 
                usuarios.alias as name_from,
                usuarios.img_src  as avatar_to, 
                usuarios.alias as name_to  
FROM pms 
LEFT JOIN usuarios ON usuarios.id = pms.id_from OR usuarios.id = pms.id_to

可行，但每个_to和_from的名称和图像都相同

那么，去这里的路是什么？

Answer 1

是的，加入同一个表将需要唯一的别名来表示表名，例如

select 
p.id,
p.id_from, 
p.id_to, 
p.fecha, 
p.leida, 
u1.img_src  as avatar_from, 
u1.alias as name_from,
u2.img_src  as avatar_to, 
u2.alias as name_to  
from pms p
LEFT JOIN usuarios u1 ON u1.id = p.id_from
LEFT JOIN usuarios u2 ON u2.id = p.id_to

Answer 2

如果您两次使用同一张表，则需要使用不同的别名

SELECT  pms.id, 
        pms.id_from, 
        pms.id_to, 
        pms.fecha, 
        pms.leida, 
        u_from.img_src  as avatar_from, 
        u_from.alias as name_from,
        u_to.img_src  as avatar_to, 
        u_to.alias as name_to 
FROM pms 
LEFT JOIN usuarios u_from ON u_from.id = pms.id_from
LEFT JOIN usuarios u_to ON u_to.id = pms.id_to