Postgres 17

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-- 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);

-- B is empty

insert into C values (10), (11), (12);

CREATE TABLE

CREATE TABLE

CREATE TABLE

INSERT 0 2

INSERT 0 3

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

a
b
c

1
null
10

1
null
11

1
null
12

2
null
10

2
null
11

2
null
12

SELECT 6

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

a
b
c

1
null
null

2
null
null

SELECT 2

select * from
  (select * from A full join B on 1=0) AB full join C on 1 = 0;

a
b
c

null
null
10

null
null
11

null
null
12

1
null
null

2
null
null

SELECT 5

select *
  from A full join (select * from B full join C on 1=0) BC on 1 = 0;

a
b
c

null
null
10

null
null
11

null
null
12

1
null
null

2
null
null

SELECT 5

a	b	c
1	null	null
2	null	null

a	b	c
null	null	10
null	null	11
null	null	12
1	null	null
2	null	null

a	b	c
null	null	10
null	null	11
null	null	12
1	null	null
2	null	null