mysql - 将两个表(具有1-M关系)连接其中第二个表需要 'flattened'到一行

Question

给定下表:

学生

+----+-------+
| id | Name  |
+----+-------+
| 1  | Chris |
| 2  | Joe   |
| 3  | Jack  |
+----+-------+

注册

+---------------+------------+-----------+----------+
| enrollment_id | student_id | course_id | complete |
+---------------+------------+-----------+----------+
| 1             | 1          | 55        | true     |
| 2             | 1          | 66        | true     |
| 3             | 1          | 77        | true     |
| 4             | 2          | 55        | true     |
| 5             | 2          | 66        | false    |
| 6             | 3          | 55        | false    |
| 7             | 3          | 66        | true     |
+---------------+------------+-----------+----------+

我想要以下内容

+----+-------+-----------+-----------+-----------+
| id | Name  | Course 55 | Course 66 | Course 77 |
+----+-------+-----------+-----------+-----------+
| 1  | Chris | true      | true      | true      |
| 2  | Joe   | true      | false     | NULL      |
| 3  | Jack  | false     | true      | NULL      |
+----+-------+-----------+-----------+-----------+

注意 1: 我知道 mysql 不能有动态列(如果我错了请纠正我!)所以我对查询开头很满意:

SELECT id, name, course_55, course_66, course_77 etc...

我对此很满意，因为类(class)数量是固定的(准确地说是 4 门)。 理想情况下我希望它是动态的；也就是说，不必在 SELECT 子句中手动编写每门类(class)。

注2:这需要纯mysql——我不想求助于PHP。

数据库目前有 10000 多名学生，注册人数为 10000+ * 4(因为正好有 4 门类(class)，每个学生都在所有 4 个模块中)。

注意 3: Student.user_id 已编入索引，enrollment.enrollment_id、enrollment.student_id 和 enrollment.course_id 也已编入索引。

Answer 1

select s.id,s.name,
max(case when e.course_id = 55 then complete else null end) as c55,
max(case when e.course_id = 66 then complete else null end) as c66,
max(case when e.course_id = 77 then complete else null end) as c77
from student as s
left join enrollment as e
on s.id = e.student_id
group by s.id

@克里斯。使用存储过程，您甚至可以在不知道列数的情况下创建动态数据透视表。这是链接

http://forum.html.it/forum/showthread.php?s=&threadid=1456236

我在意大利论坛上对类似问题的回答。有一个完整的示例可以帮助您理解背后的逻辑。 :)

编辑。使用 MYSQL DYNAMIC VIEW 更新

这是我的开始转储:

/*Table structure for table `student` */

drop table if exists `student`;

create table `student` (
  `id` int(10) unsigned not null auto_increment,
  `name` varchar(50) default null,
  primary key (`id`)
) engine=myisam;

/*Data for the table `student` */

insert  into `student`(`id`,`name`) values (1,'chris');
insert  into `student`(`id`,`name`) values (2,'joe');
insert  into `student`(`id`,`name`) values (3,'jack');

drop table if exists enrollment;

create table `enrollment` (
  `enrollment_id` int(11) auto_increment primary key,
  `student_id` int(11) default null,
  `course_id` int(11) default null,
  `complete` varchar(50) default null
) engine=myisam auto_increment=8 default charset=latin1;

/*Data for the table `enrollment` */

insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (1,1,55,'true');
insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (2,1,66,'true');
insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (3,1,77,'true');
insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (4,2,55,'true');
insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (5,2,66,'false');
insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (6,3,55,'false');
insert  into `enrollment`(`enrollment_id`,`student_id`,`course_id`,`complete`) values (7,3,66,'true');

这是动态 View 的存储过程:

delimiter //
drop procedure if exists dynamic_view//
create procedure dynamic_view()
begin
declare finish int default 0;
declare cid int;
declare str varchar(10000) default "select s.id,s.name,";
declare curs cursor for select course_id from enrollment group by course_id;
declare continue handler for not found set finish = 1;
open curs;
my_loop:loop
fetch curs into cid;
if finish = 1 then
leave my_loop;
end if;
set str = concat(str, "max(case when e.course_id = ",cid," then complete else null end) as course_",cid,",");
end loop;
close curs;
set str = substr(str,1,char_length(str)-1);
set @str = concat(str," from student as s
            left join enrollment as e
            on s.id = e.student_id
            group by s.id");
prepare stmt from @str;
execute stmt;
deallocate prepare stmt;
-- select str;
end;//
delimiter ;

现在让我们调用它

mysql> call dynamic_view();
+----+-------+-----------+-----------+-----------+
| id | name  | course_55 | course_66 | course_77 |
+----+-------+-----------+-----------+-----------+
|  1 | chris | true      | true      | true      |
|  2 | joe   | true      | false     | NULL      |
|  3 | jack  | false     | true      | NULL      |
+----+-------+-----------+-----------+-----------+
3 rows in set (0.00 sec)

Query OK, 0 rows affected (0.05 sec)

现在我们插入另外两条包含两个不同类(class)的记录:

insert  into `enrollment`(`student_id`,`course_id`,`complete`) values (1,88,'true');
insert  into `enrollment`(`student_id`,`course_id`,`complete`) values (3,99,'true');

我们还记得这个过程。这是结果:

mysql> call dynamic_view();
+----+-------+-----------+-----------+-----------+-----------+-----------+
| id | name  | course_55 | course_66 | course_77 | course_88 | course_99 |
+----+-------+-----------+-----------+-----------+-----------+-----------+
|  1 | chris | true      | true      | true      | true      | NULL      |
|  2 | joe   | true      | false     | NULL      | NULL      | NULL      |
|  3 | jack  | false     | true      | NULL      | NULL      | true      |
+----+-------+-----------+-----------+-----------+-----------+-----------+
3 rows in set (0.00 sec)

Query OK, 0 rows affected (0.02 sec)

就是这样。 :)