如何写一个查询获得免费座位在旅馆

Question

我创建了一个旅馆的数据库，我已经建立了SQL Server中的所有表，现在我正在尝试编写正弦查询。我想获得带有免费座位的房间号码。

我有表Students,Contracts,Rooms。

Contracts具有柱Student它引用在Students和Contracts的StudentId具有引用Rooms表的RoomId列Room。 RoomId是房间号码。

Room也有一列NumberSeats这是一个房间的座位总数。所以我认为这足以理解我的问题并撰写查询。

我想获得带有空闲座位的房间号码。我不知道如何实现这个，我是SQL中的新成员，所以我所尝试的一切都没有成功。你有什么想法？谢谢。

Answer 1

如果您的合同表中提供了由其ID提及的所有学生，则只需引用合同和房间即可提取数据。您可以通过连接两个表，计算每间客房的学生人数和增加的条件为having子句尝试：

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

我准备这个例子给你。 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  |