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

On Wed, Jan 6, 2021 at 3:11 AM Stefan Houtzager <stefan.houtzager@gmail.com> 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.