只需在where子句中使用条件，并在on子句中输入1=1。如下所示：

SELECT * FROM employee LEFT OUTER JOIN department ON ( 1=1) 
where employee.dpt_cd =department.dpt_cd OR 
      (employee.dpt_cd ='' AND employee.dpt_rg_cd= employee.dpt_rg_cd )

您可以将查询重写为两个具有联合的select，如下所示：

select * from employee left outer join department on (employee.dpt_cd =department.dpt_cd)
union all
select * from employee left outer join department on (employee.dpt_rg_cd = employee.dpt_rg_cd) where employee.dpt_cd ='';

这可能是一个非常慢的查询，但应该会产生您想要的结果。

要解决配置单元中的多个等式问题，请使用半左连接ie

select x.*
from employee x
LEFT SEMI JOIN  department   b on (x.buyer_id= b.id )
LEFT SEMI JOIN  department   c on (x.seller_id= c.id )