mysql多表查询的几个小例子

首先,内连结查询。

mysql> SELECT student.student_id, student.name, score.score,absence.date FROM student INNER JOIN score INNER JOIN absence INNER JOIN grade_event ON student.student_id = absence.student_id AND student.student_id = score.student_id AND score.event_id = grade_event.event_id AND grade_event.date = absence.date;
+------------+------+-------+------------+
| student_id | name | score | date       |
+------------+------+-------+------------+
|          1 | mio  |    99 | 2012-10-01 |
|          2 | yum  |    54 | 2012-10-01 |
|          3 | hehe |    22 | 2012-10-01 |
|          4 | kaka |    77 | 2012-10-01 |
|          1 | mio  |    77 | 2012-10-02 |
|          1 | mio  |    88 | 2012-10-03 |
|          1 | mio  |    99 | 2012-10-04 |
|          2 | yum  |    88 | 2012-10-04 |
+------------+------+-------+------------+
8 rows in set (0.00 sec)

统计学生参加考试的次数：

mysql> SELECT student.student_id, student.name, COUNT(absence.date) AS absences FROM student INNER JOIN absence ON student.student_id = absence.student_id GROUP BY student.student_id;
+------------+------+----------+
| student_id | name | absences |
+------------+------+----------+
|          1 | mio  |        4 |
|          2 | yum  |        3 |
|          3 | hehe |        1 |
|          4 | kaka |        2 |
+------------+------+----------+
4 rows in set (0.00 sec)

他按照student_id列出来了，但是有个id为5的家伙一次也没参加，就不会显示出来。
所以可以用LEFT JOIN左关联来代替普通的关联操作，MYSQL将为第一个数据表里的每一个中选数据生成一个输出行：

mysql> SELECT student.student_id, student.name, COUNT(absence.date) AS absences FROM student LEFT JOIN absence ON student.student_id = absence.student_id GROUP BY student.student_id;
+------------+------+----------+
| student_id | name | absences |
+------------+------+----------+
|          1 | mio  |        4 |
|          2 | yum  |        3 |
|          3 | hehe |        1 |
|          4 | kaka |        2 |
|          5 | ye   |        0 |
+------------+------+----------+
5 rows in set (0.04 sec)

子查询的例子，把一条sql语句嵌套在另一个SQL语句中：

mysql> SELECT * FROM student WHERE student_id NOT IN (SELECT student_id FROM absence);
+------+-----+------------+
| name | sex | student_id |
+------+-----+------------+
| ye   | M   |          5 |
+------+-----+------------+
1 row in set (0.00 sec)

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