Postgres 17

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

-- https://kb.databasedesignbook.com/posts/sql-joins/

create table A (a integer not null primary key, extra integer not null);

create table B (b integer not null primary key, extra integer not null);

create table C (c integer not null primary key, extra integer not null);

insert into A values (1, 1), (2, 1), (3, 2);

insert into C values (10, 1), (11, 1), (12, 1), (13, 2);

CREATE TABLE

CREATE TABLE

CREATE TABLE

INSERT 0 3

INSERT 0 4

select a, b, c from
  (select A.a, B.b, A.extra as ae from A left join B on A.extra = B.extra) AB left join C on AB.ae = C.extra;

a
b
c

1
null
10

2
null
10

1
null
11

2
null
11

1
null
12

2
null
12

3
null
13

SELECT 7

select a, b, c
  from A left join (select B.b, C.c, B.extra as be from B left join C on B.extra = C.extra) BC on A.extra = BC.be;

a
b
c

1
null
null

2
null
null

3
null
null

SELECT 3

a	b	c
1	null	null
2	null	null
3	null	null