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

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Stefan Houtzager

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

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

David G Johnston

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

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.

Stefan Houtzager

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

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

David G Johnston

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

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.

Stefan Houtzager

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

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

David G Johnston

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

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.

Stefan Houtzager

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

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