Thread: [GENERAL] Concatenating NULL with JSONB value return NULL

PostgreSQL 9.6.1

Hi

I have a NULL-able JSONB type column and want to perform upsert,
concatenating with the existing value.

The query looks like (campaigns and facts columns are JSONB type, in the
below) :

INSERT INTO Fan  (oid, campaigns, facts)  VALUES (189,'{"campaign-id":
"12345"}','{"attended": false}') ON CONFLICT (oid)
DO UPDATE SET campaigns = EXCLUDED.campaigns, facts = fan.facts ||
EXCLUDED.facts RETURNING *;

And this does not work when the existing JSONB type column has NULL
value. For example:

select NULL::JSONB || '{"A": "b"}'::JSONB;

I would expect the above returns '{"A": "b"}', but PostgreSQL does not
work as I expected.

What's the best way to make concatenating with NULL returns the
right-hand side value?
(One way I can think of is, giving a default value of '{}' instead of
NULL, but I'd like to know any alternatives)


Thanks

- Jong-won

On 12/18/2016 2:52 PM, Jong-won Choi wrote:
>
> I have a NULL-able JSONB type column and want to perform upsert,
> concatenating with the existing value.

NULL does not mean 'NO' value in SQL it means UNKNOWN value. sort of
like the 'indeterminate' in math.

maybe you want a NOT NULL json value that you set to ''  or something
when its empty.


--
john r pierce, recycling bits in santa cruz

Melvin Davidson <melvin6925@gmail.com> writes:
> On Sun, Dec 18, 2016 at 6:08 PM, John R Pierce <pierce@hogranch.com> wrote:
>> On 12/18/2016 2:52 PM, Jong-won Choi wrote:
>>> I have a NULL-able JSONB type column and want to perform upsert,
>>> concatenating with the existing value.

>> NULL does not mean 'NO' value in SQL it means UNKNOWN value. sort of like
>> the 'indeterminate' in math.

> Have you tried using CASE?

> INSERT INTO Fan  (oid, campaigns, facts)  VALUES (189,'{"campaign-id":
> "12345"}','{"attended": false}')
> ON CONFLICT (oid)
> DO UPDATE SET campaigns = EXCLUDED.campaigns,
>               CASE WHEN fan.facts is NULL
>                    THEN facts = EXCLUDED.facts
>                    ELSE facts = fan.facts || EXCLUDED.facts
>                END
>     RETURNING *;

Another option is COALESCE:

...
DO UPDATE SET campaigns = EXCLUDED.campaigns,
              facts = COALESCE(fan.facts, '{}'::jsonb) || EXCLUDED.facts
...

I'd argue though that if you think this is okay, then you're abusing
NULL; that's supposed to mean "unknown", not "known to be empty".
It would be better to initialize the column to '{}' to begin with.

            regards, tom lane

Thanks Tom, Melvin, and John!


@John, I keep forgetting the semantic differences between my programming
language and PostgreSQL.


I will go for Tom's COALESCE than Melvin's, purely for less typing.


Thanks again, all!


- Jong-won


On 19/12/16 11:46, Tom Lane wrote:
> Melvin Davidson <melvin6925@gmail.com> writes:
>> On Sun, Dec 18, 2016 at 6:08 PM, John R Pierce <pierce@hogranch.com> wrote:
>>> On 12/18/2016 2:52 PM, Jong-won Choi wrote:
>>>> I have a NULL-able JSONB type column and want to perform upsert,
>>>> concatenating with the existing value.
>>> NULL does not mean 'NO' value in SQL it means UNKNOWN value. sort of like
>>> the 'indeterminate' in math.
>> Have you tried using CASE?
>> INSERT INTO Fan  (oid, campaigns, facts)  VALUES (189,'{"campaign-id":
>> "12345"}','{"attended": false}')
>> ON CONFLICT (oid)
>> DO UPDATE SET campaigns = EXCLUDED.campaigns,
>>                CASE WHEN fan.facts is NULL
>>                     THEN facts = EXCLUDED.facts
>>                     ELSE facts = fan.facts || EXCLUDED.facts
>>                 END
>>      RETURNING *;
> Another option is COALESCE:
>
> ...
> DO UPDATE SET campaigns = EXCLUDED.campaigns,
>                facts = COALESCE(fan.facts, '{}'::jsonb) || EXCLUDED.facts
> ...
>
> I'd argue though that if you think this is okay, then you're abusing
> NULL; that's supposed to mean "unknown", not "known to be empty".
> It would be better to initialize the column to '{}' to begin with.
>
>             regards, tom lane
>
>