postgresql - 内连接sql的结合性和交换性

Question

假设我有这个模式。

Boats
_____
bid
bname

Reserves
________
sid
bid
date

Sailors
_______
sid
sname

我知道内部联接应该是关联的和交换的，但我真的不明白为什么。

给定查询:

SELECT sname, bname
FROM (Sailors NATURAL INNER JOIN Boats) NATURAL INNER JOIN Reserves

我认为这应该返回 null，因为 Sailors 和 Boats 没有公共(public)字段，而:

SELECT sname, bname
FROM (Sailors NATURAL INNER JOIN Reserves) NATURAL INNER JOIN Boats

应该返回水手的名字和他们预定的船的名字。

请告诉我为什么内部联接应该同时具有交换性和关联性。

谢谢!

Answer 1

SELECT sname, bname
FROM (Sailors NATURAL INNER JOIN Boats) NATURAL INNER JOIN Reserves

I am thinking that this should return null since Sailors and Boats have no common fields, . . .

在 PostgreSQL 中 natural join两个没有公共(public)列的表之间的行为类似于交叉连接。

create table boats (
  bid integer primary key,
  bname varchar(15)
);

create table sailors (
  sid integer primary key,
  sname varchar(15) 
);

insert into boats values (1, 'One'), (2, 'Two'), (3, 'Three');
insert into sailors values (1, 'One'), (2, 'Two'), (3, 'Three');

SELECT sname, bname
FROM (Sailors NATURAL INNER JOIN Boats);

sname    bname--One      OneOne      TwoOne      ThreeTwo      OneTwo      TwoTwo      ThreeThree    OneThree    TwoThree    Three