MySql查询select from 两个表与left join on的区别

直接看sql：

mysql>  select  *  from  a;
+ ------+------+
| id   | col  |
+ ------+------+
|    1 |   11 |
|    2 |   12 |
|    3 |   13 |
+ ------+------+
3  rows  in  set  (0.00 sec)

mysql>  select  *  from  b;
+ ------+------+
| id   | col  |
+ ------+------+
|    2 |   22 |
|    3 |   23 |
|    5 |   25 |
+ ------+------+
3  rows  in  set  (0.00 sec)

mysql>
mysql>  select  *  from  a,b  where  a.id=b.id;
+ ------+------+------+------+
| id   | col  | id   | col  |
+ ------+------+------+------+
|    2 |   12 |    2 |   22 |
|    3 |   13 |    3 |   23 |
+ ------+------+------+------+
2  rows  in  set  (0.08 sec)

mysql>  select  *  from  a  left  join  b  on  a.id=b.id;
+ ------+------+------+------+
| id   | col  | id   | col  |
+ ------+------+------+------+
|    1 |   11 |  NULL  |  NULL  |
|    2 |   12 |    2 |   22 |
|    3 |   13 |    3 |   23 |
+ ------+------+------+------+
3  rows  in  set  (0.00 sec)

mysql>