comparing two fields that may be null - postgresql

Question

Is there a comparison operator where a.unitnum = b.unitnum would be true if both a.unitnum and b.unitnum are null? Seems that a.unitnum IS b.unitnum is invalid

Answer 1

yes, there is 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

yes, there is, but it is recomended to not use it. here is sample:
t=# select null = null;
?column?
----------
(1 row)
t=# set transform_null_equals = on;
SET
t=# select null = null;
?column?
----------
t
(1 row)
UPDATE: apparently would work only for comparison column = NULL, not column = column:
t=# with s as (select null::int a, null::int b) select a <> b from s;
?column?
----------
(1 row)
so the shortest comparison would be coalesce:
t=# with s as (select null::int a, null::int b) select coalesce(a,b,0) = 0 from s;
?column?
----------
t
(1 row)

Answer 3

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 4

No but you can use a.unitnum = b.unitnum or (a.unitnum is null and b.unitnum is null)

Answer 5

If you need to handle all cases:
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
Then you may want to use this expression:
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));
Here you can test how it works:
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);
P.S.
Just use IS NOT DISTINCT FROM from the accepted answer... It works the same but shorter.