Question

我有桌子：

Student(sID, firstName, lastName, email, cgpa)
Course(cNum, name, dept, credit)
Offering(oID, cNum, dept, year, term, instructor)
Took(sID, oID, grade)

我正在尝试完成问题：

Find all courses for the term 2017F and the current enrolment

我目前有这个查询来获取每个课程的注册学生人数：

SELECT Took.oID, COUNT(*) AS enrolment
FROM Took
GROUP BY Took.oID
HAVING COUNT(*) > 0

嵌套在此语句中，以获取我希望其注册人数正确的课程：

SELECT oID
FROM Offering
WHERE Offering.year = 2017
AND Offering.term = 'F'

这两个查询都嵌套在其中以将所有内容绑定在一起：

SELECT DISTINCT Offering.cNum, Course.name, (I WOULD LIKE COUNT(*) AS enrolment HERE)
FROM Offering NATURAL JOIN Course
WHERE Offering.oID IN (
            SELECT oID
            FROM Offering
            WHERE Offering.year = 2017
            AND Offering.term = 'F'
            AND oID IN (
                    SELECT Took.oID, COUNT(*) AS enrolment
                    FROM Took
                    GROUP BY Took.oID
                    HAVING COUNT(*) > 0))
GROUP BY Offering.cNum, Course.name;

我的问题是，如何将结果COUNT（*）AS注册从最远的嵌套查询传递到初始查询，以便它可以显示在结果投影中？（这是家庭作业）

Answer 1

如果我正确理解，您可以尝试在from和JOIN中使用子查询，而不是where子查询。

然后您可以从子查询中获取count列。

SELECT DISTINCT Offering.cNum, Course.name,t1.enrolment
FROM Offering 
JOIN (
    SELECT Took.oID, 
        COUNT(*) AS enrolment
    FROM Took
    GROUP BY Took.oID
    HAVING COUNT(*) > 0
) t1 on t1.oID = Offering.oID
NATURAL JOIN Course
WHERE Offering.year = 2017 AND Offering.term = 'F'

Answer 2

尝试一下

SELECT c.*
    , (
        SELECT COUNT(*)
        FROM Took
        WHERE oID = o.oID
    ) AS theCount
FROM Course c
JOIN Offering o ON o.cNum = c.cNum
WHERE o.year = 2017 AND o.term = 'F'

Answer 3

可能是这个

SELECT Course.name, Course.cNum, count(*) as enrolment
FROM Course
JOIN Offering ON Course.cNum = Offering.cNum
JOIN Took ON Offering.oID = Took.oID
WHERE Offering.year = 2017
AND Offering.term = 'F'
GROUP BY Course.name, Course.cNum
HAVING count(*) > 0;

SQL嵌套聚合函数

3 个答案: