多个加入如何工作？它的工作顺序是什么？ [英] How multiple join work? In What sequence it works?

问题描述

假设我有4个表说： -

1. A

2. B

3. C

4. D

Suppose I have 4 Tables say:-
1. A
2. B
3. C
4. D

create table A
(id int,
sal int);

insert into A  values(1,100);
insert into A  values(2,200);

create table B
(id int,
sal int);

insert into B  values(1,100);

create table C
(id int,
sal int);

insert into C  values(1,100);
insert into C  values(2,200);

create table D
(id int,
sal int);

insert into D  values(1,100);
insert into D  values(2,200);

和我执行以下查询说： -

and I execute following query say: -

select a.id as AID,a.sal as Asal,b.id as BID ,b.sal as Bsal , 
c.id as CID , c.sal as CSal ,d.id as Did , d.sal
from A as a,B as b , C as c ,D as d;

如何4表的笛卡尔积是否有效？

How does the cartesian product of 4 table works??

推荐答案

笛卡儿积是一种数学运算，它从多个集合中返回集合（或产品集）。也就是说，对于集合A和B，笛卡尔乘积A×B是所有有序对（a，b）的集合，其中a∈A和b∈B。

A Cartesian product is a mathematical operation which returns a set (or product set) from multiple sets. That is, for sets A and B, the Cartesian product A × B is the set of all ordered pairs (a, b) where a ∈ A and b ∈ B.

A = {1,2}; B = {3,4} 
A × B = {1,2} × {3,4} = {(1,3), (1,4), (2,3), (2,4)}

在您的案例集中，其笛卡尔积如下。

In your case set and its cartesian product will be as follows.

A = {(1,100), (2,200)}
B = {(1,100)}
C = {(1,100), (2,200)}
D = {(1,100), (2,200)}

1	100	1	100	1	100	1	100
1	100	1	100	1	100	2	200
1	100	1	100	2	200	1	100
1	100	1	100	2	200	2	200
2	200	1	100	1	100	1	100
2	200	1	100	1	100	2	200
2	200	1	100	2	200	1	100
2	200	1	100	2	200	2	200

这篇关于多个加入如何工作？它的工作顺序是什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！