如何编写查询以获得旅馆的免费座位

Question

我为旅馆创建了一个数据库，我已经在 SQL Server 中构建了所有表，现在我正在尝试编写正弦查询。我想获取有空位的房间号。

我有表Students、Contracts、Rooms。

Contracts 有一个列 Student 引用 Students 中的 StudentId 和 Contracts 有列 Room 引用 Rooms 表的 RoomId。 RoomId 是房间号。

Room 也有一个列NumberSeats，它是房间中的座位总数。所以我认为这足以理解我的问题并编写查询。

我想获取其中有可用空位的房间号。我不知道如何实现这一点，我是 SQL 新手，所以我尝试的一切都没有结果。你有什么想法？谢谢。

Answer 1

如果您的“合同”表中有 ID 提及的所有学生，则可以通过仅引用“合同”和“房间”来提取数据。您可以尝试将两个表连接起来，计算每个房间的学生人数，然后在 having 子句中添加条件：

SELECT b.RoomId,
  b.NumberSeats,
  COUNT(a.Student)
FROM Rooms b
LEFT JOIN Contracts a
ON a.Room=b.RoomId
GROUP BY b.RoomId,
  b.NumberSeats
HAVING b.NumberSeats>COUNT(a.Student);

Answer 2

我已经为您准备了这个示例。 ddl + 查询 -->

create table student
(id number,
name varchar2(100));

create table room
(id  number,
name varchar2(100));

create table contract
(student_id number,
room_id number);

create table number_seats
(room_id number,
number_seats number);

insert into student values(1, 'Jon');
insert into student values(2, 'George');
insert into student values(3, 'Ian');
insert into student values(4, 'Alex');
insert into student values(5, 'Mary');

insert into room values(1, 'room1');
insert into room values(2, 'room2');
insert into room values(3, 'room3');
insert into room values(4, 'room4');
insert into room values(5, 'room5');

insert into number_seats values(1, 1);
insert into number_seats values(2, 2);
insert into number_seats values(3, 3);
insert into number_seats values(4, 2);
insert into number_seats values(5, 1);

insert into contract values(1, 1);
insert into contract values(2, 2);
insert into contract values(3, 3);
insert into contract values(4, 4);
insert into contract values(5, 4);

select * from room;
select * from student;
select * from contract;
select * from number_seats;


select cont.room_id, num_seats.number_seats - count(*)
from student st 
join contract cont on st.id=cont.student_id
join room ro on ro.id=cont.room_id
join number_seats num_seats on num_seats.room_id=ro.id
group by cont.room_id, num_seats.number_seats 
union
select ro.id, num_seats.number_seats
from room ro left join contract co on ro.id=co.room_id
join number_seats num_seats on num_seats.room_id=ro.id
where co.room_id is null
order by 1;

结果是

|roomNumber|freeSeats|
|    1     |  0      |
|    2     |  1      |
|    3     |  2      |
|    4     |  0      |
|    5     |  1      |