sql - 更简单的 sql 查询

Question

我有以下表格

• 学生（sid，姓名，年龄）
• 课程（cid，cname，持续时间）
• 注册（sid，cid，日期）

查找所有参加过 sid = 1 的课程的 sid。

select sid from enroll where cid in (select cid from enroll where sid=1) 
group by sid having count(*)=(select count(*) from enroll where sid=1) 
minus (select sid from student where sid=1);

1. 第一个 count(*) 指的是什么？
2. 是否有更简单的查询版本？

Answer 1

第一个COUNT(*)是指属于特定的行数sid。

重新格式化：

SELECT sid
  FROM enroll
 WHERE cid IN (SELECT cid FROM enroll WHERE sid=1) 
 GROUP by sid
HAVING COUNT(*) = (SELECT COUNT(*) FROM enroll WHERE sid=1) 
MINUS (SELECT sid FROM student WHERE sid=1);

您当然可以使用以下方法避免 MINUS：

SELECT sid
  FROM enroll
 WHERE cid IN (SELECT cid FROM enroll WHERE sid=1)
   AND sid != 1
 GROUP by sid
HAVING COUNT(*) = (SELECT COUNT(*) FROM enroll WHERE sid=1) 

Answer 2

试试这个

SELECT s.sid
FROM student s, enroll e
WHERE s.sid = e.sid
AND e.cid IN (SELECT cid FROM course WHERE sid=1)
AND s.sid <> 1