MySQL Multi-Table Query method (join)

Table A records the following:
AID Anum
1 a20050111
2 a20050112
3 a20050113
4 a20050114
5 a20050115

Table B records the following:
BID bname
1 2006032401
2 2006032402
3 2006032403
4 2006032404
8 2006032408

Create these two table SQL statements as follows:
CREATE TABLE A
AID Int (1) auto_increment PRIMARY KEY,
Anum Char (20)
)
CREATE TABLE B (
BID Int (1) not NULL auto_increment PRIMARY KEY,
Bname Char (20)
)

INSERT into a
VALUES (1, ' a20050111 '), (2, ' a20050112 '), (3, ' a20050113 '), (4, ' a20050114 '), (5, ' a20050115 ');

INSERT into B
VALUES (1, ' 2006032401 '), (2, ' 2006032402 '), (3, ' 2006032403 '), (4, ' 2006032404 '), (8, ' 2006032408 ');

The experiment is as follows:
1.left join (left join)

The SQL statements are as follows:
SELECT * from a
Left JOIN b
On A.aid =b.bid

The results are as follows:
AID Anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
5 a20050115 NULL NULL
(The number of rows affected is 5 rows)

Result Description:
The left join is based on the records of Table A, a can be regarded as the right table, and B can be regarded as left table.
In other words, the records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).
The low-record of table B is null.

2.right join (right join)

The SQL statements are as follows:
SELECT * from a
Right joing b
On a.aid = B.bid

The results are as follows:
AID Anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032404
NULL NULL 8 2006032408
(The number of rows affected is 5 rows)

Result Description:
Looking closely, you will find that the result of the left join is exactly the opposite, this time it is based on the right table (B), where a table is not enough to fill with null.

3.inner Join (equal join or inner join)

The SQL statements are as follows:
SELECT * from a
INNER JOIN b
On A.aid =b.bid

Equivalent to the following SQL sentence:
SELECT *
From a A B
WHERE A.aid = B.bid

The results are as follows:
AID Anum BID bname
1 a20050111 1 2006032401
2 a20050112 2 2006032402
3 a20050113 3 2006032403
4 a20050114 4 2006032

MySQL Multi-Table Query method (join)