概念
INNER JOIN(内连接)：获取两个表中字段匹配关系的记录。也就是只会返回共有的内容。
LEFT JOIN(左连接)：获取左表所有记录，即使右表没有对应匹配的记录。
RIGHT JOIN(右连接)： 获取右表所有记录，即使左表没有对应匹配的记录。
示例
先在数据库中建立两张表student和score，具体内容如下：
【student】
mysql> select * from student;
--------------
select * from student
--------------
+----+---------------------+------+-------+------------+-----------+
| id | name | sex | birth | department | address |
+----+---------------------+------+-------+------------+-----------+
| 1 | RooneyMara | F | 1985 | Psychology | American |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia |
| 3 | EllenPage | F | 1987 | Music | Canada |
| 4 | TomHolland | M | 1996 | CS | England |
| 5 | ScarlettJohansson | F | 1984 | Music | American |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England |
| 7 | EvaGreen | F | 1980 | Math | France |
+----+---------------------+------+-------+------------+-----------+
7 rows in set (0.00 sec)
【score】
mysql> select * from score;
--------------
select * from score
--------------
+----+--------+------------+-------+
| id | stu_id | c_name | grade |
+----+--------+------------+-------+
| 1 | 1 | Psychology | 98 |
| 2 | 1 | Music | 80 |
| 3 | 2 | Psychology | 65 |
| 4 | 2 | CS | 88 |
| 5 | 3 | CS | 95 |
| 6 | 4 | Psychology | 70 |
| 7 | 4 | Music | 92 |
| 8 | 5 | Music | 94 |
| 9 | 6 | Psychology | 90 |
| 10 | 6 | CS | 85 |
| 11 | 8 | Music | 91 |
+----+--------+------------+-------+
11 rows in set (0.00 sec)
内连接
查询student表中的所有个人信息及score表中的c_name,grade
mysql> select a.*,c_name,grade from student a join score b on a.id=b.stu_id;
--------------
select a.*,c_name,grade from student a join score b on a.id=b.stu_id
--------------
+----+---------------------+------+-------+------------+-----------+------------+-------+
| id | name | sex | birth | department | address | c_name | grade |
+----+---------------------+------+-------+------------+-----------+------------+-------+
| 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 |
| 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 |
| 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 |
| 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 |
| 4 | TomHolland | M | 1996 | CS | England | Music | 92 |
| 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 |
+----+---------------------+------+-------+------------+-----------+------------+-------+
10 rows in set (0.00 sec)
以上语句等价于：
mysql> select a.*,c_name,grade from student a,score b where a.id=b.stu_id;
--------------
select a.*,c_name,grade from student a,score b where a.id=b.stu_id
--------------
+----+---------------------+------+-------+------------+-----------+------------+-------+
| id | name | sex | birth | department | address | c_name | grade |
+----+---------------------+------+-------+------------+-----------+------------+-------+
| 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 |
| 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 |
| 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 |
| 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 |
| 4 | TomHolland | M | 1996 | CS | England | Music | 92 |
| 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 |
+----+---------------------+------+-------+------------+-----------+------------+-------+
10 rows in set (0.00 sec)
左连接
student表中id为7的数据，在score中没有对应的内容。所以最后一条查询结果c_name,grade对应内容为null。
mysql> select a.*,c_name,grade from student a left join score b on a.id=b.stu_id;
--------------
select a.*,c_name,grade from student a left join score b on a.id=b.stu_id
--------------
+----+---------------------+------+-------+------------+-----------+------------+-------+
| id | name | sex | birth | department | address | c_name | grade |
+----+---------------------+------+-------+------------+-----------+------------+-------+
| 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 |
| 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 |
| 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 |
| 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 |
| 4 | TomHolland | M | 1996 | CS | England | Music | 92 |
| 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 |
| 7 | EvaGreen | F | 1980 | Math | France | NULL | NULL |
+----+---------------------+------+-------+------------+-----------+------------+-------+
11 rows in set (0.00 sec)
右连接
score表中id为11的数据，在student中没有对应的内容，所以最后一条查询结果id,name,sex等对应内容为null。
mysql> select a.*,c_name,grade from student a right join score b on a.id=b.stu_id;
--------------
select a.*,c_name,grade from student a right join score b on a.id=b.stu_id
--------------
+------+---------------------+------+-------+------------+-----------+------------+-------+
| id | name | sex | birth | department | address | c_name | grade |
+------+---------------------+------+-------+------------+-----------+------------+-------+
| 1 | RooneyMara | F | 1985 | Psychology | American | Psychology | 98 |
| 1 | RooneyMara | F | 1985 | Psychology | American | Music | 80 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | Psychology | 65 |
| 2 | ChrisHemsworth | M | 1983 | CS | Australia | CS | 88 |
| 3 | EllenPage | F | 1987 | Music | Canada | CS | 95 |
| 4 | TomHolland | M | 1996 | CS | England | Psychology | 70 |
| 4 | TomHolland | M | 1996 | CS | England | Music | 92 |
| 5 | ScarlettJohansson | F | 1984 | Music | American | Music | 94 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | Psychology | 90 |
| 6 | BenedictCumberbatch | M | 1976 | Psychology | England | CS | 85 |
| NULL | NULL | NULL | NULL | NULL | NULL | Music | 91 |
+------+---------------------+------+-------+------------+-----------+------------+-------+
11 rows in set (0.00 sec)