Select certain keys from array of objects - mysql

Question

Say I have a JSON column foo where each value is an array of objects, i.e:
[{"date": somedate, "value": somevalue, "othervaluesidontneed":...},
{"date": somedate, "value": somevalue, "othervaluesidontneed":...},
{"date": somedate, "value": somevalue, "othervaluesidontneed":...},...]
I want to select this column but for each row, to only include the keys date and value, so the returned value is:
[{"date": somedate, "value": somevalue},
{"date": somedate, "value": somevalue},
{"date": somedate, "value": somevalue},...]
Is this possible?

Answer 1

A solution would be to use json_table() (available in MySQ 8.0 only) to expand the array as a set of rows, and then generate a new array of objects that contain only the requested keys with json_arrayagg():
select
json_arrayagg(json_object( 'date', tt.date, 'value', tt.value)) new_js
from
mytable t,
json_table(
js,
"$[*]"
columns(
date datetime path "$.date",
value int path "$.value"
)
) as tt
group by t.id
This requires that some column can be used to uniquely identify a row in the initial table: I called that column id.
Demo on DB Fiddle:
with mytable as (
select 1 id, '[
{ "date": "2019-01-01", "value": 1, "othervaluesidontneed": 12 },
{ "date": "2019-01-02", "value": 2, "othervaluesidontneed": 55 },
{ "date": "2019-01-03", "value": 3, "othervaluesidontneed": 72}
]' js
)
select
json_arrayagg(json_object( 'date', tt.date, 'value', tt.value)) new_js
from
mytable t,
json_table(
js,
"$[*]"
columns(
date datetime path "$.date",
value int path "$.value"
)
) as tt
group by t.id
| new_js |
| :----------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [{"date": "2019-01-01 00:00:00.000000", "value": 1}, {"date": "2019-01-02 00:00:00.000000", "value": 2}, {"date": "2019-01-03 00:00:00.000000", "value": 3}] |