coalesce(json_agg [..] filter where [..], '[]' in left join returning null

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coalesce(json_agg [..] filter where [..], '[]' in left join returning null

Stefan Houtzager

This query

select

         json_agg(
            json_build_object(
                'vat_cat',      
                (select row_to_json(_) from
                    (select vc.id, vc.descr, vc.expense, vc.version, vat_percentage) as _
                )
            )  
        )    
        from vat_cat vc
        left join (
            select
                vatcat_id,
                COALESCE(
                json_agg(
                       (select row_to_json(_) from
                           (select vp.percentage, vp.version, json_build_object(
                               'lower', lower(vp.validity),
                               'upper', upper(vp.validity),
                               'lower_inc', lower_inc(vp.validity),
                               'upper_inc', upper_inc(vp.validity)
                           ) validity)
                        as _)
                ) FILTER (WHERE vp.vatcat_id IS NOT NULL), '[]'::JSON) vat_percentage
            from vat_percentage vp
            group by vatcat_id
        ) vp on vc.id = vp.vatcat_id`

Gives as one of it's vat_cat (with no related vat_percentage records) results

{ "vat_cat": { "id": 10, "descr": "nonsense", "expense": true, "version": 1, "vat_percentage": null } } 

How do I get the query right so that it display [] instead of null?


--
Kind regards,

Stefan Houtzager

Houtzager ICT consultancy & development

www.linkedin.com/in/stefanhoutzager
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Re: coalesce(json_agg [..] filter where [..], '[]' in left join returning null

David G Johnston
On Wed, Jan 6, 2021 at 3:11 AM Stefan Houtzager <[hidden email]> wrote:

                COALESCE(

                json_agg(
                       (select row_to_json(_) from
                           (select vp.percentage, vp.version, json_build_object(
                               'lower', lower(vp.validity),
                               'upper', upper(vp.validity),
                               'lower_inc', lower_inc(vp.validity),
                               'upper_inc', upper_inc(vp.validity)
                           ) validity)
                        as _)
                ) FILTER (WHERE vp.vatcat_id IS NOT NULL), '[]'::JSON) vat_percentage
{ "vat_cat": { "id": 10, "descr": "nonsense", "expense": true, "version": 1, "vat_percentage": null } } 

How do I get the query right so that it display [] instead of null?


Use COALESCE(NULLIF(..., 'null'::json), '[]'::json); NULLIF converts the JSON null into SQL NULL which the COALESCE then replaces with a empty json array.

David J.

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Re: coalesce(json_agg [..] filter where [..], '[]' in left join returning null

Stefan Houtzager
Thanks David!

When I run the following in pgadmin I get the error that follows at the bottom. Any idea how to get the query right?

select
         json_agg(
         (select row_to_json(_) from
                  (select vc.id, vc.descr, vc.expense, vc.version, vat_percentage) as _
              )    
         )    
        from vat_cat vc
        left join (
            select
                vatcat_id,
                COALESCE(NULLIF(
                  json_agg(
                        (select row_to_json(_) from
                            (select vp.percentage, vp.version, json_build_object(
                                'lower', lower(vp.validity),
                                'upper', upper(vp.validity),
                                'lower_inc', lower_inc(vp.validity),
                                'upper_inc', upper_inc(vp.validity)
                            ) validity)
                          as _)
                  ),
                'null'::json), '[]'::json) vat_percentage
            from vat_percentage vp
            group by vatcat_id
        ) vp on vc.id = vp.vatcat_id

ERROR: operator does not exist: json = json LINE 11: COALESCE(NULLIF( ^ HINT: No operator matches the given name and argument types. You might need to add explicit type casts. SQL state: 42883 Character: 306  

On Wed, Jan 6, 2021 at 4:10 PM David G. Johnston <[hidden email]> wrote:
On Wed, Jan 6, 2021 at 3:11 AM Stefan Houtzager <[hidden email]> wrote:

                COALESCE(

                json_agg(
                       (select row_to_json(_) from
                           (select vp.percentage, vp.version, json_build_object(
                               'lower', lower(vp.validity),
                               'upper', upper(vp.validity),
                               'lower_inc', lower_inc(vp.validity),
                               'upper_inc', upper_inc(vp.validity)
                           ) validity)
                        as _)
                ) FILTER (WHERE vp.vatcat_id IS NOT NULL), '[]'::JSON) vat_percentage
{ "vat_cat": { "id": 10, "descr": "nonsense", "expense": true, "version": 1, "vat_percentage": null } } 

How do I get the query right so that it display [] instead of null?


Use COALESCE(NULLIF(..., 'null'::json), '[]'::json); NULLIF converts the JSON null into SQL NULL which the COALESCE then replaces with a empty json array.

David J.



--
Kind regards,

Stefan Houtzager

Houtzager ICT consultancy & development

www.linkedin.com/in/stefanhoutzager
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Re: coalesce(json_agg [..] filter where [..], '[]' in left join returning null

David G Johnston
On Wed, Jan 6, 2021 at 8:59 AM Stefan Houtzager <[hidden email]> wrote:

ERROR: operator does not exist: json = json LINE 11: COALESCE(NULLIF( ^ HINT: No operator matches the given name and argument types. You might need to add explicit type casts. SQL state: 42883 Character: 306  


Right, json doesn't implement equality.  You will need to cast to and compare text values.

David J.

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Re: coalesce(json_agg [..] filter where [..], '[]' in left join returning null

Stefan Houtzager
Thanks again David. I am very novice, and tried the following which does not give an error but returns null instead of []
select
         json_agg(
         (select row_to_json(_) from
                  (select vc.id, vc.descr, vc.expense, vc.version, vat_percentage) as _
              )    
         )    
        from vat_cat vc
        left join (
            select
                vatcat_id,
                COALESCE(NULLIF(
                  json_agg(
                        (select row_to_json(_) from
                            (select vp.percentage, vp.version, json_build_object(
                                'lower', lower(vp.validity),
                                'upper', upper(vp.validity),
                                'lower_inc', lower_inc(vp.validity),
                                'upper_inc', upper_inc(vp.validity)
                            ) validity)
                          as _)
                  )::text,
                'null'::text), '[]'::text) vat_percentage
            from vat_percentage vp
            group by vatcat_id

        ) vp on vc.id = vp.vatcat_id

On Wed, Jan 6, 2021 at 5:10 PM David G. Johnston <[hidden email]> wrote:
On Wed, Jan 6, 2021 at 8:59 AM Stefan Houtzager <[hidden email]> wrote:

ERROR: operator does not exist: json = json LINE 11: COALESCE(NULLIF( ^ HINT: No operator matches the given name and argument types. You might need to add explicit type casts. SQL state: 42883 Character: 306  


Right, json doesn't implement equality.  You will need to cast to and compare text values.

David J.



--
Kind regards,

Stefan Houtzager

Houtzager ICT consultancy & development

www.linkedin.com/in/stefanhoutzager
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Re: coalesce(json_agg [..] filter where [..], '[]' in left join returning null

David G Johnston
On Wed, Jan 6, 2021 at 9:32 AM Stefan Houtzager <[hidden email]> wrote:
Thanks again David. I am very novice, and tried the following which does not give an error but returns null instead of []

Please don't top-post here.

The general concept should work.  You may at least wish to experiment in a simple query to get the feel for the mechanics.  With respect to this query I'd probably need to have a working query to play with (and time) to give a more precise answer.

David J.
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Re: coalesce(json_agg [..] filter where [..], '[]' in left join returning null

Stefan Houtzager
On Wed, Jan 6, 2021 at 5:38 PM David G. Johnston <[hidden email]> wrote:

Please don't top-post here.

The general concept should work.  You may at least wish to experiment in a simple query to get the feel for the mechanics.  With respect to this query I'd probably need to have a working query to play with (and time) to give a more precise answer.

David J.

Thanks David,

I do not get what you suggest working within the left-join. What does work is the following:

select json_agg(_)
from (
    select
        vc.id,
   vc.descr,
   vc.expense,
   vc.version,
        (select coalesce(json_agg(_), '[]')
         from (
             select
                 vp.percentage,
    vp.version,  
    json_build_object(
                     'lower', lower(vp.validity),
                     'upper', upper(vp.validity),
                     'lower_inc', lower_inc(vp.validity),
                     'upper_inc', upper_inc(vp.validity)
                  ) validity
                  from vat_percentage vp
                  where vp.vatcat_id = vc.id
                ) _
         ) as percentages
       from vat_cat vc
    group by vc.id
) _

--
Kind regards,

Stefan Houtzager

Houtzager ICT consultancy & development

www.linkedin.com/in/stefanhoutzager