比较两个可能为null的字段

Question

如果a.unitnum和b.unitnum都为空，是否存在a.unitnum = b.unitnum为true的比较运算符？似乎a.unitnum IS b.unitnum无效

Answer 1

是的，有IS DISTINCT FROM and IS NOT DISTINCT FROM

postgres=# \pset null ****
Null display is "****".
postgres=# select null = null;
┌──────────┐
│ ?column? │
╞══════════╡
│ ****     │
└──────────┘
(1 row)

postgres=# select null is not distinct from null;
┌──────────┐
│ ?column? │
╞══════════╡
│ t        │
└──────────┘
(1 row)

postgres=# select 10 = null;
┌──────────┐
│ ?column? │
╞══════════╡
│ ****     │
└──────────┘
(1 row)

postgres=# select 10 is distinct from null;
┌──────────┐
│ ?column? │
╞══════════╡
│ t        │
└──────────┘
(1 row)

postgres=# select 10 is not distinct from null;
┌──────────┐
│ ?column? │
╞══════════╡
│ f        │
└──────────┘
(1 row)

postgres=# select 10 is not distinct from 20;
┌──────────┐
│ ?column? │
╞══════════╡
│ f        │
└──────────┘
(1 row)

Answer 2

IF(a.unitnum IS null AND b.unitnum IS null)
THEN
   RAISE NOTICE 'unitum field is null in both a and b tables'
ELSE
   RAISE NOTICE 'unitum field is not null in at least one a or b tables'
END IF;

Answer 3

是，there is，但是it is recomended to not use it。这是样本：

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

(1 row)

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

更新：显然只适用于比较column = NULL，而不是列=列：

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

(1 row)

因此最短的比较将是合并的：

t=# with s as (select null::int a, null::int b) select coalesce(a,b,0) = 0 from s;
 ?column?
----------
 t
(1 row)

Answer 4

否但您可以使用a.unitnum = b.unitnum或（a.unitnum为null且b.unitnum为null）

Answer 5

如果您需要处理所有情况：

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

那么你可能想使用这个表达式：

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));

在这里你可以测试它是如何工作的：

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 只需使用已接受答案中的 IS NOT DISTINCT FROM ......它的工作原理相同但更短。