Postgresql 比较两个可能为空的字段

如果a.unitnum和b.unitnum都为null，是否存在a.unitnum=b.unitnum将为

true

的比较运算符？似乎a.unitnum是b.unitnum无效

否，但您可以使用a.unitnum=b.unitnum或（a.unitnum为null，b.unitnum为null）

是，但是。以下是示例：

t=# select null = null;
 ?column?
----------

(1 row)

t=# set transform_null_equals = on;
SET
t=# select null = null;
 ?column?
----------
 t
(1 row)

更新：显然只能用于比较

column=NULL

，而不是column=column:

t=# with s as (select null::int a, null::int b) select a <> b from s;
 ?column?
----------

(1 row)

是的，有

---

如果您需要处理所有案件：

a.unitnum is null        b.unitnum is null
a.unitnum is null        b.unitnum is not null
a.unitnum is not null    b.unitnum is null
a.unitnum is not null    b.unitnum is not null

然后，您可能需要使用以下表达式：

select * 
from a, b
where 
((a.unitnum is not null) and (b.unitnum is not null) and (a.unitnum = b.unitnum)) or
((a.unitnum is null) and (b.unitnum is null));

在这里，您可以测试它的工作原理：

SELECT 
  ((a is not null) and (b is not null) and (a = b)) or
  ((a is null) and (b is null))
FROM (VALUES (null,null)
           , (null,1)
           , (1,null)
           , (1,1)
           , (1,2)
       ) t1 (a, b);

附言。

---

仅使用

与公认答案没有区别。。。它的工作原理相同，但更短。
kha！它没有比较两列：）我得到的合并类型text和integer无法匹配，我怀疑它是否可能匹配-不确定是否存在
运算符text=integer
。无论如何，要进行比较，您需要
合并（text\u colun，int\u column:：text，'null'）='null'
注意
null或false
返回
null
null或true
返回
true
。您还应该知道
null和true
返回
null
null和false
返回
false
。
a.unitnum is null        b.unitnum is null
a.unitnum is null        b.unitnum is not null
a.unitnum is not null    b.unitnum is null
a.unitnum is not null    b.unitnum is not null

select * 
from a, b
where 
((a.unitnum is not null) and (b.unitnum is not null) and (a.unitnum = b.unitnum)) or
((a.unitnum is null) and (b.unitnum is null));

SELECT 
  ((a is not null) and (b is not null) and (a = b)) or
  ((a is null) and (b is null))
FROM (VALUES (null,null)
           , (null,1)
           , (1,null)
           , (1,1)
           , (1,2)
       ) t1 (a, b);