对于联接表中不存在的行，如何将空值更新为0？

编程问答 2022-08-16

问题描述

我有2张桌子。

requests
id|code|count|


oldrequests
id|code|count|

我需要从 oldrequests 更新 requests 中的 count 。我试图用Coalesce（oldrequests.count，0）更新NULL count 值的主要问题是这样的：

update request req
set count = coalesce(oldreq.count,0)
from oldrequests oldreq
where req.code = oldreq.code

但是请求中的NULL值未更新。 然后我尝试将NULL值设置为0，如下所示：

update request req
set count = 0
from oldrequests oldreq
where req.code = oldreq.code and oldreq.id is null

但这对我也没有帮助。 如何更新值和NULL值？

解决方法

我的意思是问题是oldrequests表中行的更新计数不匹配。我可以建议采用以下解决方案：

通过requests更新0表中的所有计数，并在更新后匹配oldrequests表中的行计数，例如：

update requests set count = 0;

update requests req
set count = coalesce(oldreq.count,0)
from oldrequests oldreq
where req.code = oldreq.code;

为不匹配的行设置了0的另一种方法：

update requests set count = 0
where (select code from oldrequests where oldrequests.code = requests.code) is null;

db<>fiddle

或使用not exist子句的另一个版本：

update requests set count = 0
where not exists (
    select code from oldrequests where oldrequests.code = requests.code
);

db<>fiddle

joinjoinpostgresql