Re: update without SET ?

On Wed, Feb 3, 2016 at 2:51 PM, David G. Johnston <
david(dot)g(dot)johnston(at)gmail(dot)com> wrote:

> On Wed, Feb 3, 2016 at 11:49 AM, Michael Moore <michaeljmoore(at)gmail(dot)com>
> wrote:
>
>> I want to UPDATE a table but I will not know until run time which columns
>> will be updated. I would like to do something like this:
>> update mytest t (SELECT * FROM
>> json_populate_record(null::mytest,'{"key":22,"header":44,"ident":66,"static01":"this","static02":"that"}'));
>>
>> In other words, I will be receiving a json document on an input
>> parameter. The columns named in the json document can be a sub set of those
>> that exist in the table.
>>
>>
> If a key is defined but has a value of null what do you want to do?
>
> ​not fully tested...​
>
> UPDATE mytest
> SET col1 = COALESCE(src.col1, mytest.col1)
> ​FROM (SELECT * FROM json_populate_record(...)) src;​
>
> json_populate_record outputs the full structure of mytest regardless of
> its input so simply match every column and use COALESCE to choose between
> the value in the json and the value already on the table.
>
> Aside from the null json value issue that should work - if not you
> probably need to describe your problem in more detail.
>
>
​Modification thought to deal with JSON null values on existing keys​:

UPDATE mytest
SET col1 = (CASE WHEN src.json_value ? 'col1' THEN src.col1 ELSE
mytest.col1 END)
FROM (
SELECT *
FROM json_populate_record(json_value)
CROSS JOIN
SELECT json_value AS source_json
) src WHERE mytest.key = src.key;

​David J.​