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在数据库 'college2' 中有 3 个表：'student, course & enrolment' 和一个 (1) VIEW:'enrolment_status'，它是使用以下命令创建的：

CREATE VIEW enrolment_status AS
SELECT code, COUNT(id)
FROM enrolment
GROUP BY code;

解释“课程、注册和注册状态”的命令导致：

mysql> EXPLAIN course;
+---------------+-------------+------+-----+---------+-------+
| Field         | Type        | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+-------+
| code          | char(8)     | NO   | PRI | NULL    |       |
| name          | varchar(90) | YES  | MUL | NULL    |       |
| max_enrolment | char(2)     | YES  |     | NULL    |       |
+---------------+-------------+------+-----+---------+-------+
3 rows in set (0.09 sec)

mysql> explain enrolment;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | char(6) | YES  | MUL | NULL    |       |
| code  | char(8) | YES  | MUL | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.02 sec)

mysql> explain enrolment_status;
+-----------+------------+------+-----+---------+-------+
| Field     | Type       | Null | Key | Default | Extra |
+-----------+------------+------+-----+---------+-------+
| code      | char(8)    | YES  |     | NULL    |       |
| COUNT(id) | bigint(21) | NO   |     | 0       |       |
+-----------+------------+------+-----+---------+-------+
2 rows in set (0.18 sec)

“course”表中的“max_enrolment”列是每门课程允许的最大学生人数，例如 10 或 20。“enrolment_status”视图（不是表）中的“count(id)”列是每门课程注册的实际学生人数. 'enrolment' TABLE 中的 'id' 列是在课程中注册的学生 ID。

这是我的问题：我想要“剩余座位数”，这是“max_enrolment”列和“count(id)”列之间的差异。“剩余座位数”可以是独立的表格或视图，也可以是添加到上述任何表格的列。我怎样才能做到这一点：

我尝试了许多命令，包括以下命令，

CREATE VIEW seats_left AS (
    SELECT course.code, course.max_enrolment - enrolment_status.count
    FROM course, enrolment_status
    WHERE course.code = enrolment_status.code);

...这给了我以下错误消息：

ERROR 1054 (42S22): Unknown column 'enrolment_status.count' in 'field list'

mysql> SELECT*FROM enrolment_status;
+----------+-----------+
| code     | COUNT(id) |
+----------+-----------+
| COMP9583 |         7 |
| COMP9585 |         9 |
| COMP9586 |         7 |
| COMP9653 |         7 |
| COMP9654 |         7 |
| COMP9655 |         8 |
| COMP9658 |         7 |
+----------+-----------+
7 rows in set (0.00 sec)

mysql> SELECT code, max_enrolment FROM course;
+----------+---------------+
| code     | max_enrolment |
+----------+---------------+
| COMP9583 | 10            |
| COMP9585 | 15            |
| COMP9586 | 15            |
| COMP9653 | 12            |
| COMP9654 | 10            |
| COMP9655 | 12            |
| COMP9658 | 12            |
+----------+---------------+
7 rows in set (0.00 sec)

+----------+---------------------+
| code     | max_enrolment - cnt |
+----------+---------------------+
| COMP9583 |                   9 |
| COMP9585 |                  14 |
| COMP9586 |                  14 |
| COMP9653 |                  11 |
| COMP9654 |                   9 |
| COMP9655 |                  11 |
| COMP9658 |                  11 |
+----------+---------------------+
7 rows in set (0.09 sec)
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2 回答 2

2

尝试在视图中使用首字母缩略词。

CREATE VIEW enrolment_status AS
SELECT code, COUNT(id) count
FROM enrolment
GROUP BY code;

然后你应该能够做到这一点：

CREATE VIEW seats_left AS (
SELECT course.code, course.max_enrolment - enrolment_status.count
FROM course, enrolment_status
WHERE course.code = enrolment_status.code);

如果您无法更改视图，则必须在查询中使用完全相同的名称：

CREATE VIEW seats_left AS (
SELECT course.code, course.max_enrolment - enrolment_status.'count(id)'
FROM course, enrolment_status
WHERE course.code = enrolment_status.code);
于 2017-12-14T01:06:01.253 回答
0

尝试这个：

SELECT b.`code`,max_enrolment - cnt from
(select `code`, cnt from
(select count(1) as cnt,`code` from enrolment_status
GROUP BY `code`) as a) as a

LEFT JOIN

(SELECT code,max_enrolment from course) as b
on a.`code` = b.`code`

您可以更改left join为right join

于 2017-12-14T05:58:08.750 回答