How multiple join work? In What sequence it works?

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

SQL

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);

and I execute following query say: -

SQL

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;

How does the cartesian product of 4 table works??

Posted 12-Jul-13 0:13am

Dhage Kanchan

Updated 12-Jul-13 1:42am

Sudhakar Shinde

v3

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[no name] 12-Jul-13 6:35am

it takes first rows of A, B , and C and product with all D, next first of A and B and second row of C, and product with all D, and so on, finally you will have 2x2x2x1 = 8 rows

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Sudhakar Shinde · Answer 1 · 2013-07-12T01:38:00

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)}

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