Detailed analysis of the usage of left join, right join, and inner join in SQL

Table A records as follows: 
aID aNum 
1 a11 
2 a22 
3 a33 
4 a44 
5 a55

Table B records as follows: 
bID bName 
1 b11 
2 b22 
3 b33 
4 b44 
8 b88

The SQL statements for creating these two tables are 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, 'a11' ); 
commit; 
.....

INSERT INTO b 
VALUES ( 1, 'b11' ); 
commit; 
.....

1.left join (left join)

sql语句如下: 
SELECT * FROM a 
LEFT JOIN b 
ON a.aID =b.bID

The results are as follows: 
aID aNum bID bName 
1 a11 1 b11 
2 a22 2 b22 
3 a33 3 b33 
4 a44 4 b44 
5 a55 NULL NULL 
(the number of rows affected is 5 rows)

Result description: 
left join is based on the records of table A, A can