如何理解SQL中的连接？

Create table If Not Exists student_info (id int, name varchar(255));
Create table If Not Exists student_score (id int, score int);
Truncate table student_info;
insert into student_info (id, name) values ('1', 'Wang');
insert into student_info (id, name) values ('2', 'Alice');
insert into student_info (id, name) values ('3', 'Allen');
Truncate table student_score;
insert into student_score (id, score) values ('1', '65');
insert into student_score (id, score) values ('2', '70');
insert into student_score (id, score) values ('3', '75');

mysql> SELECT * FROM student_score, student_info;
+------+-------+------+-------+
| id   | score | id   | name  |
+------+-------+------+-------+
|    3 |    75 |    1 | Wang  |
|    2 |    70 |    1 | Wang  |
|    1 |    65 |    1 | Wang  |
|    3 |    75 |    2 | Alice |
|    2 |    70 |    2 | Alice |
|    1 |    65 |    2 | Alice |
|    3 |    75 |    3 | Allen |
|    2 |    70 |    3 | Allen |
|    1 |    65 |    3 | Allen |
+------+-------+------+-------+
9 rows in set (0.00 sec)

mysql> SELECT * FROM student_info JOIN student_score;
+------+-------+------+-------+
| id   | name  | id   | score |
+------+-------+------+-------+
|    3 | Allen |    1 |    65 |
|    2 | Alice |    1 |    65 |
|    1 | Wang  |    1 |    65 |
|    3 | Allen |    2 |    70 |
|    2 | Alice |    2 |    70 |
|    1 | Wang  |    2 |    70 |
|    3 | Allen |    3 |    75 |
|    2 | Alice |    3 |    75 |
|    1 | Wang  |    3 |    75 |
+------+-------+------+-------+
9 rows in set (0.00 sec)