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);

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

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

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

insert into B values (2);

-- insert into C values (2), (11), (12);

CREATE TABLE

CREATE TABLE

CREATE TABLE

INSERT 0 2

INSERT 0 1

select * from
  (select * from A left join B on 1=1) AB left join C on 1 = 1;

a
b
c

1
2
null

2
2
null

SELECT 2

select *
  from A left join (select * from B left join C on 1=1) BC on 1 = 1;

a
b
c

1
2
null

2
2
null

SELECT 2

select * from
  (select * from A cross join B) AB cross join C;

a
b
c

SELECT 0

select *
  from A cross join (select * from B cross join C) BC;

a
b
c

SELECT 0

a	b	c
1	2	null
2	2	null

a	b	c
1	2	null
2	2	null