多次加倍 JOIN 到同一个 table

Question

示例 1（工作正常，选择 assignedagent）：

select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin,
cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype,
u.userlastname as assignedagentname,
"" as answeringagentname from callrecord cr
left join (agentrecord ar left join users u on ar.agentkey=u.userkey)
on (cr.callid=ar.callid and cr.assignedagent=ar.sequencenumber);

示例 2（工作正常，选择 answeringagent）：

select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin,
cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype,
"" as assignedagentname,
u.userlastname as answeringagentname from callrecord cr
left join (agentrecord ar left join users u on ar.agentkey=u.userkey)
on (cr.callid=ar.callid and cr.answeringagent=ar.sequencenumber);

示例 3（产生错误，同时选择两者）：

select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin,
cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype,
u1.userlastname as assignedagentname,
u2.userlastname as answeringagentname from callrecord cr
left join (agentrecord ar left join users u1 on ar.agentkey=u1.userkey)
on (cr.callid=ar.callid and cr.assignedagent=ar.sequencenumber)
left join (agentrecord ar left join users u2 on ar.agentkey=u2.userkey)
on (cr.callid=ar.callid and cr.answeringagent=ar.sequencenumber);

示例 1 和示例 2 工作正常，returns 结果正确。在示例 3 中，我遵循了有关在该论坛中找到的多个连接的别名的建议，但没有成功。我通过 ODBC 驱动程序使用 Informix DB。

Answer 1

尝试删除括号并确保所有别名都是唯一的：

select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin,
       cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype,
       u1.userlastname as assignedagentname,
       u2.userlastname as answeringagentname
from callrecord cr left join
     agentrecord ar1
     on cr.callid = ar1.callid and
        cr.assignedagent = ar1.sequencenumber left join
     users u1
     on ar.agentkey = u1.userkey left join
     agentrecord ar2 
     on cr.callid = ar2.callid and 
        cr.answeringagent = ar2.sequencenumber left join
     users u2 on ar2.agentkey = u2.userkey

多次加倍 JOIN 到同一个 table

Multiple doubled JOIN to the same table

sql

informix