将两个表（具有1-M关系）联接在一起，其中第二个表需要“展平”到一行
给出下表：
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
+----+-------+-----------+-----------+-----------+| id | Name | Course 55 | Course 66 | Course 77 |
+----+-------+-----------+-----------+-----------+
| 1 | Chris | true | true | true |
| 2 | Joe | true | false | NULL |
| 3 | Jack | false | true | NULL |
+----+-------+-----------+-----------+-----------+
我知道mysql不能有动态列（如果我错了，请纠正我！），因此我对查询的开头感到满意：
SELECT id, name, course_55, course_66, course_77 etc...我对此感到满意，因为课程的数量是固定的（准确地说是4门）。 我希望它是动态的。也就是说，不必手动在SELECT子句中编写每个课程。
这需要纯mysql-我不想求助于PHP。
该数据库目前可容纳10000多名学生，并具有10000 + * 4个注册（因为正好有4门课程，每个学生都在全部4个模块中）。
Student.user_id和enrollment.enrollment_id，enrollment.student_id和enrollment.course_id也已建立索引。
回答：
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
我在一个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了解背后的逻辑。:)
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');
这是动态视图的存储过程：
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 ;
现在叫它
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，我们插入另外两个具有两个不同课程的记录：
insert into `enrollment`(`student_id`,`course_id`,`complete`) values (1,88,'true');insert into `enrollment`(`student_id`,`course_id`,`complete`) values (3,99,'true');
我们回想一下程序。结果如下：
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)
就这样。:)
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