Postgres-与空时间戳比较

Question

您好，这不是一个真正的问题，我只想了解原因：

在postgres 9中

this_.lastModified<=NULL

评估为真，为什么？lastModified是没有时区的时间戳

我认为将其解释为“ this_.lastModified <= 0”会更符合逻辑，如果0是最低日期而lastModified是正常日期，则应评估为false

完整的查询如下所示

select 
this_.*
from Entity this_ 
inner join DEntity d2_ on this_.device=d2_.id 
inner join u u1_ on this_.learner=u1_.userID 
inner join LMEntity m3_ on this_.method=m3_.id 
where u1_.userID='XXXX' and not (d2_.hardwareID='muh' and this_.timestamp='2013-08-02 00:00:00' and m3_.id=0 and this_.lastModified<=NULL)

Answer 1

this_.lastModified<=NULL总是求和null，在这种情况下，您的where子句有效：

where u1_.userID='XXXX' and not (d2_.hardwareID='muh' and this_.timestamp='2013-08-02 00:00:00' and m3_.id=0 and null)

如果这里所有比较：

d2_.hardwareID='muh' and this_.timestamp='2013-08-02 00:00:00' and m3_.id=0

评估为“ true”，整个​​子句评估为true：

where u1_.userID='XXXX' and not (true and null)

true and null 评估为 null

where u1_.userID='XXXX' and not null

not null 评估为 null

where u1_.userID='XXXX' and null

如果u1_.userID='XXXX'相等true，则 u1_.userID='XXXX' and null得出null

和where null等于where false。

总之，整个

where u1_.userID='XXXX' and not (d2_.hardwareID='muh' and this_.timestamp='2013-08-02 00:00:00' and m3_.id=0 and this_.lastModified<=NULL)

将评估到null，如果u1_.userID='XXXX'所有的d2_.hardwareID='muh' and this_.timestamp='2013-08-02 00:00:00' and m3_.id=0给true