MySQL multi-table connection problem

MySQL connections have outer links (left and right joins), inner joins, and cross joins (Cartesian product).

The statement to create a table based on mysql is as follows:

drop table abc,cdef;
CREATE TABLE abc (
    a VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'a',
    b VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'b',
    c VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'c'
    
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='abc';


CREATE TABLE cdef (
    c VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'c',
    d VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'd',
    e VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'e',
    f VARCHAR(20) NOT NULL DEFAULT '0' COMMENT 'f'  
)ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='cdef';




insert into abc (a,b,c) values 
('a1','b1','c1'),
('a1','b1','c1'),
('a5','b5','c5'),
('a2','b2','c2');
insert into cdef (c,d,e,f) values 
('c1','d1','e1','f1'),
('c3', 'd3', 'e3', 'f3'),
('c2','d2','e2','f2'),
('c2','d2','e2','f2');

• one. Cross join:   A cross join without a where clause will result in a Cartesian product of the joined tables. The total number of records is equal to the product of the total number of data in the two tables.

-- Cartesian cross join
select *
from abc ,cdef;

result:

a1 b1 c1 c1 d1 e1 f1
a1 b1 c1 c1 d1 e1 f1
a5 b5 c5 c1 d1 e1 f1
a2 b2 c2 c1 d1 e1 f1
a1 b1 c1 c3 d3 e3 f3
a1 b1 c1 c3 d3 e3 f3
a5 a2 b5 c5 e3 _ _ _ b2 c2 c3 d3 e3 f3 a1

b1 c1 c2 d2 e2 f2
a1 b1 c1 c2 d2 e2 f2
a5 b5 c5 c2 d2 e2 f2
a2 b2 c2 c2 d2 e2 f2
a1 b1 c1 c2 d2 e2 f2
a1 b1 c1 c2 d2 e2 f2
a5 b2 c2 c2 f2 _ _ _ _ c2 c2 d2 e2 f2

• two. Inner joins: There are three types of inner joins: natural joins, equijoins, and non-equijoins

a. Natural join:  Use the equal = operator in the join condition to compare the column values ​​of the joined columns, but delete the duplicate columns in the join table

select *
from abc natural join cdef;

c1 a1 b1 d1 e1 f1
c1 a1 b1 d1 e1 f1
c2 a2 b2 d2 e2 f2
c2 a2 b2 d2 e2 f2

b. Equi-join: Use on or where to add the join condition  equal to the = operator to compare the column values ​​of the joined columns without deleting duplicate columns

select a.*,d.*
from  abc a, cdef d
where a.c = d.c;

a1 b1 c1c1d1 e1 f1 a1 b1 c1 c1 d1 e1 f1 a2 b2 c2 c2 d2 e2 f2 a2 b2 c2 c2 d2 e2 f2

c. Non-equivalent join:    In the join condition, other comparison operators can be used to compare the column values ​​of the joined columns, such as: <, >, !=, etc.

select a.*,d.*
from  abc a, cdef d
where a.c != d.c;

result:

a2 b2 c2 c1 d1 e1 f1 a1 b1 c1 c3 d3 e3 f3 a1 b1 c1 c3 d3 e3 f3 a5 b5 c5 c3 d3 e3 f3 a2 b2 c2 c3 d3 e3 f3 a1 b1 c1 c2 d2 e2 f2 a1 b2 c1 e2 _ _ _ _ b5 c5 c2 d2 e2 f2 a1







b1 c1 c2 d2 e2 f2
a1 b1 c1 c2 d2 e2 f2
a5 b5 c5 c2 d2 e2 f2

• three. External links: including left join, right join

a. Left connection: The left is the main, the right is connected to the left, and the corresponding columns on the right are all null

select a.*,d.*
from  abc a left join  cdef d on a.c = d.c;

result:

a1 b1 c1 c1 d1 e1 f1
a1 b1 c1 c1 d1 e1 f1
a5 b5 c5 (null) (null) (null) (null) a2 b2 c2 c2 d2 e2 f2 a2 b2 c2 c2 d2 e2 f2

b. Right join: The right side prevails, the left side is connected to the right, and the corresponding columns on the left side are all null

select a.*,d.*
from  abc a right join  cdef d on a.c = d.c;

result:

a1 b1 c1 c1 d1 e1 f1
a1 b1 c1 c1 d1 e1 f1
(zero) (zero) (zero) c3 d3 e3 f3
a2 b2 c2 c2 d2 e2 f2
a2 b2 c2 c2 d2 e2 f2