Postgres 13

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create  table list (firstname text,lastname text,age integer);

CREATE TABLE

insert into list 
values 
('SHARON','XAVIER',25),
('RON','PETER',17),
('KIM','BENNY',14);

INSERT 0 3

select jsonb_agg(to_jsonb(l))
from list l;

jsonb_agg

[{"age": 25, "lastname": "XAVIER", "firstname": "SHARON"}, {"age": 17, "lastname": "PETER", "firstname": "RON"}, {"age": 14, "lastname": "BENNY", "firstname": "KIM"}]

SELECT 1