Question

我在过去几天一直在尝试写一个执行以下操作的MySQL查询：

对于每个至少有一名合格教授教授课程的班级，列出有资格教授班级的教授人数。

我尝试了以下查询，但它没有提供正确的输出

select Class, Semester, p.EmpId from Qualified q, Teaches T, Professor p where q.EmpId = p.EmpId and T.EmpId = p.EmpId group by p.EmpId;

+-------+---------+-------+---------+-----------+------+-----------------------------------+
| EmpId | ClassId | EmpId | FName   | LName     | Dept | QualClass                         |
+-------+---------+-------+---------+-----------+------+-----------------------------------+
|   101 | BA101   |   101 | Bobby   | Ewing     | BA   | BA101, BA102                      |
|   102 | BA101   |   102 | Red     | Skeleton  | BA   | BA101, BA102                      |
|   101 | BA102   |   101 | Bobby   | Ewing     | BA   | BA101, BA102                      |
|   102 | BA102   |   102 | Red     | Skeleton  | BA   | BA101, BA102                      |
+-------+---------+-------+---------+-----------+------+-----------------------------------+

这些是相关表格。

CREATE TABLE Professor
(
    EmpId INTEGER,
    FName VARCHAR(10),
    LName VARCHAR(10),
    Dept VARCHAR(2),
    QualClass VARCHAR(40),
    PRIMARY KEY (EmpId)
);

CREATE TABLE Teaches
(
    Class VARCHAR(5),
    Section INTEGER,
    Semester CHAR(4),
    EmpId INTEGER,
    FOREIGN KEY (EmpId) REFERENCES Professor(EmpId)
);

CREATE TABLE Qualified
(
    EmpId INTEGER,
    ClassId VARCHAR(5)
);

以下是一些DML语句

INSERT INTO Professor (EmpId, FName, LName, Dept, QualClass) VALUES (101, 'Bobby', 'Ewing', 'BA', 'BA101, BA102');
INSERT INTO Professor (EmpId, FName, LName, Dept, QualClass) VALUES (102, 'Red', 'Skeleton', 'BA', 'BA101, BA102');
INSERT INTO Teaches (Class, Section, Semester, EmpId) VALUES ('BA101', 1, 'FA12', 101);
INSERT INTO Teaches (Class, Section, Semester, EmpId) VALUES ('BA101', 1, 'SP13', 102);
INSERT INTO Teaches (Class, Section, Semester, EmpId) VALUES ('BA101', 1, 'SU13', 101);
INSERT INTO Teaches (Class, Section, Semester, EmpId) VALUES ('BA102', 1, 'FA12', 102);
INSERT INTO Teaches (Class, Section, Semester, EmpId) VALUES ('BA102', 1, 'SP13', 101);
INSERT INTO Teaches (Class, Section, Semester, EmpId) VALUES ('BA102', 1, 'SU13', 102);
INSERT INTO Qualified (ClassId, EmpId) VALUES ('BA101', 101);
INSERT INTO Qualified (ClassId, EmpId) VALUES ('BA101', 102);
INSERT INTO Qualified (ClassId, EmpId) VALUES ('BA102', 101);
INSERT INTO Qualified (ClassId, EmpId) VALUES ('BA102', 102);

Answer 1

根据您的陈述：

对于每个至少有一名合格教授教授课程的班级，列出有资格教授班级的教授人数。

以下查询列出有资格教授课程的教授人数。这里有一个SQLFiddle来展示它的作用：

SELECT Q.ClassId, count(Q.EmpId) as "Professor Count"
FROM Qualified Q
GROUP BY ClassId

现在，如果您还需要知道哪些教授是合格的，那将是不同的查询。

无法编写MySQL查询

1 个答案: