Sql 带表条件的Where子句

我想写一个查询，它将根据卡车的状态过滤返回的行数。如果我只希望看到尚未关闭的卡车，我将从UI发送一个值

'1'

，发送一个

'2'

以查看所有卡车

select *
from invoice left outer
join truck
     on (invoice.invnum = truck.invnum)

这将返回正确的行数

问题： 我需要一个where子句，它将查看truck表的

close\u date

字段，并确定它是否为null。这里是一个问题，如果在卡车表中没有找到记录，仍然从发票表返回该行

select *
from invoice left outer
join truck
     on (invoice.invnum = truck.invnum)
where UI.value = (select close_date from truck where invoice.invnum = truck.invnum) “if no record is returned here, return the value ‘2’

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请澄清你的问题。你想做这样的事吗

select *
from 
    invoice 
     left outer join 
    truck on 
        invoice.invnum = truck.invnum
where 
    (UI.value = 1 AND truck.close_date IS NULL) OR
    (UI.value = 2)

还是你想这么做

select *
from 
    invoice 
     left outer join 
    truck on 
        invoice.invnum = truck.invnum
where UI.Value = 
  (
    SELECT CASE WHEN MAX(close_date) IS NULL THEN 2 ELSE 1 END 
    FROM truck 
    WHERE invoice.invnum = truck.invnum)

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我想你有点误解了

where

条款。或者至少在这里它能为你做些什么。您是否可以将其放入一个封装在

if

语句中的存储过程中？