Postgres 13

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CREATE TABLE aaa (
    id integer NOT NULL,
    dataval integer NULL
);

CREATE TABLE bbb (
    aaa_id integer NOT NULL,
    bbb_id integer NOT NULL,
    sort_val integer NOT NULL,
    data_val integer NOT NULL
);

INSERT INTO aaa (id,dataval) VALUES (1,NULL);
INSERT INTO aaa (id,dataval) VALUES (2,NULL);
INSERT INTO aaa (id,dataval) VALUES (3,NULL);

INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (1,1,3,10);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (1,2,2,11);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (1,3,1,12);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (2,4,6,13);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (2,5,5,14);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (2,6,4,15);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (3,7,9,16);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (3,8,8,17);
INSERT INTO bbb (aaa_id,bbb_id,sort_val,data_val) VALUES     (3,9,7,18);

CREATE TABLE

CREATE TABLE

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

select a.id, b.data_val
from aaa a
left join lateral (
    select b.data_val
    from bbb b
    where b.aaa_id = a.id
    order by sort_val
    limit 1
) b on true

id
data_val

1
12

2
15

3
18

SELECT 3

select distinct on (a.id) a.id, b.data_val
from aaa a
left join bbb b on b.aaa_id = a.id
order by a.id, b.sort_val

id
data_val

1
12

2
15

3
18

SELECT 3