Question

我有一个电子商务应用程序，逻辑简化如下：

有7个学院，每个学院有一个座位，有4个学生有兴趣入学。

以下是如何分配席位......

Akbar将在大学入学324，因为这是他的第一选择，没有竞争。现在他已退出比赛了即使他处于第二位，阿马尔也入围大学162。安东尼也在大学456获得了他的第一选择，学生打电话给＃34; shantanu＆＃34;没有在任何大学录取。

是否有返回预期结果的SQL查询？

drop table college;

create table college (id int not null, primary key (id));
insert into college values (162), (324), (456), (862), (169), (589), (489);

drop table students;
create table students (name varchar(255), marks int, first_pref int, second_pref int,third_pref int);
insert into students values ('shantanu', 67, 162, 324,456);
insert into students values ('amar', 98, 162, 862, 169);
insert into students values ('akbar', 99, 324, 162, 589);
insert into students values ('anthony', 76, 456, 489, 589);

select * from students;
+----------+-------+------------+-------------+------------+
| name     | marks | first_pref | second_pref | third_pref |
+----------+-------+------------+-------------+------------+
| shantanu |    67 |        162 |         324 |        456 |
| amar     |    98 |        162 |         862 |        169 |
| akbar    |    99 |        324 |         162 |        589 |
| anthony  |    76 |        456 |         489 |        589 |
+----------+-------+------------+-------------+------------+
4 rows in set (0.00 sec)

select name, marks, ( case 
when first_pref=162 then 1 
when second_pref = 162 then 2 
when third_pref = 162 then 3 else 0 
end ) as mypref from students having mypref > 0
order by   marks desc;
+----------+-------+--------+
| name     | marks | mypref |
+----------+-------+--------+
| akbar    |    99 |      2 |
| amar     |    98 |      1 |
| shantanu |    67 |      1 |
+----------+-------+--------+
3 rows in set (0.00 sec)

select name, marks, ( case
when first_pref=324 then 1
when second_pref = 324 then 2
when third_pref = 324 then 3 else 0
end ) as mypref from students having mypref > 0
order by  marks desc;
+----------+-------+--------+
| name     | marks | mypref |
+----------+-------+--------+
| akbar    |    99 |      1 |
| shantanu |    67 |      2 |
+----------+-------+--------+
2 rows in set (0.00 sec)

select name, marks, ( case
when first_pref=456 then 1
when second_pref = 456 then 2
when third_pref = 456 then 3 else 0
end ) as mypref from students having mypref > 0
order by  marks desc;
+----------+-------+--------+
| name     | marks | mypref |
+----------+-------+--------+
| anthony  |    76 |      1 |
| shantanu |    67 |      3 |
+----------+-------+--------+
2 rows in set (0.00 sec)

预期的输出是这样的......

Student_name college_id
akbar 324
amar 162
anthony 456
shantanu NULL

Answer 1

select name,id from students,college, (case

when first_pref,second_pref,third_pref is not null then mypref
(case
first_pref,second_pref,third_pref == null
end)
else null
end)

as mypref;

Answer 2

要获得case的学生首选：

select name,
       (case when first_pref  is not null then first_pref
             when second_pref is not null then second_pref
             when third_pref  is not null then third_pref
             else null
        end) as mypref
from students;

将客户与供应商联系起来，给予选择

2 个答案: