Database 使用oracle中的count函数显示未注册课程的学生

我必须列出学生姓名和他/她所修课程的数量（包括尚未参加任何课程的学生）

这是我5张桌子中的3张，不需要教员和课程桌子

--Student Table
CREATE TABLE Student(
Std_ID NUMBER(4) CONSTRAINT Student_ID_pk PRIMARY KEY,
Std_FN VARCHAR2(20),
Std_LN VARCHAR2(20),
Std_City VARCHAR2(20),
Std_State VARCHAR2(20),
Std_Zip NUMBER(5),
Std_Major VARCHAR2(10),
Std_Class VARCHAR2(2),
Std_GPA NUMBER(3,2) CONSTRAINT Student_GPA_cc CHECK (Std_GPA<= 4.0 AND Std_GPA>=0));

INSERT INTO Student
VALUES('101','Joe','Smith','Eau Clare','WI', '18121','IS','FR','3.0');
INSERT INTO Student
VALUES('102','Jenny','Sneider','Eau Clare','WI', '98011','IS','JR','3.2');
INSERT INTO Student
VALUES('103','Dan','Robinson','Sartell','MN', '98042','IS','JR','3.5');
INSERT INTO Student
VALUES('104','Sue','Williams','St.Cloud','MN', '56301','ACCT','SR','3.2');
-------------------------------------------------------------------------------
--Offering Table
CREATE TABLE Offering(
Offer_No NUMBER(4) CONSTRAINT Offer_No_pk PRIMARY KEY,
Course_No VARCHAR2(10) CONSTRAINT Course_No_fk REFERENCES Course(Course_No),
Off_Term VARCHAR2(7),
Off_Year NUMBER(4),
Off_Loca VARCHAR2(10),
Off_Time Varchar2(8),
Off_Day VARCHAR2(7),
Fac_SSN NUMBER(4) CONSTRAINT Fac_SSn_fk REFERENCES Faculty(Fac_ID));

INSERT INTO OFFERING
VALUES('2201', 'IS 250', 'Spring', '2000', 'BB260', '10:30am', 'MWF', '9002');
INSERT INTO OFFERING
VALUES('2202', 'IS 250', 'Spring', '1999', 'BB118', '8:00am', 'TTH', '9002');
INSERT INTO OFFERING
VALUES('2203', 'IS 350', 'Fall', '2001', 'BB260', '9:30am', 'TTH', '9001');
INSERT INTO OFFERING
VALUES('2204', 'IS 351', 'Fall', '2001', 'BB315', '12:30pm', 'TTH', '9003');
INSERT INTO OFFERING
VALUES('1101', 'ACCT 291', 'Fall', '2000', 'BB320', '12:30pm', 'MWF', '9010');
INSERT INTO OFFERING
VALUES('2205', 'IS 443', 'Fall', '2002', 'BB216', '12:30pm', 'MWF', '9003');
---------------------------------------------------------------------------------
--Enrollment Table
CREATE TABLE Enrollment(
Std_ID NUMBER(4) CONSTRAINT Enroll_Std_ID_fk REFERENCES Student(std_ID),
Offer_No NUMBER(4) CONSTRAINT Enroll_Offer_No_fk REFERENCES Offering(Offer_No),
Enr_Grade Char(1) CONSTRAINT Enroll_grade_cc CHECK (Enr_Grade IN('A','B','C','D','F')),
CONSTRAINT Enroll_pk PRIMARY KEY (Std_ID,Offer_No));

INSERT INTO ENROLLMENT
VALUES('101', '2201', 'A');
INSERT INTO ENROLLMENT
VALUES('101', '2203', 'B');
INSERT INTO ENROLLMENT
VALUES('102', '2203', 'C');
INSERT INTO ENROLLMENT
VALUES('103', '2203', 'B');
INSERT INTO ENROLLMENT
VALUES('103', '2201', 'C');
INSERT INTO ENROLLMENT
VALUES('103', '1101', 'B');

---

您需要一个

OUTER

连接到这里。没有注册的学生将从注册表返回

NULL

值，因此计数为0。您修改的查询是

SELECT  Count(Enrollment.Std_ID) as Num_Courses, Std_FN, Std_LN
    FROM Enrollment, Student
    WHERE Student.Std_ID = Enrollment.Std_ID(+)
    Group by Student.Std_ID, Std_FN, Std_LN
    Having Count(*)>= 0;

---

您的查询有一个内部联接，这意味着

Std\u ID

必须在两个表中，否则该行将不包括在内。换句话说，没有课程的学生不在统计的结果集中

改为使用左连接，并使用

注册.Std\u ID

。如果该学生没有匹配的

注册

行，则

注册.Std_ID

将为空，因此它不会被计算，这将导致该学生为零-这就是您想要的

还有一件事：我很确定Oracle会希望您在

SELECT

子句中包含

Std\u ID

，如果您按它分组的话

SELECT COUNT(Enrollment.Std_ID) AS Num_Courses, Student.Std_ID, Std_FN, Std_LN
FROM Student
LEFT JOIN Enrollment ON Student.Std_ID = Enrollment.Std_ID
GROUP BY Student.Std_ID, Std_FN, Std_LN

SELECT COUNT(Enrollment.Std_ID) AS Num_Courses, Student.Std_ID, Std_FN, Std_LN
FROM Student
LEFT JOIN Enrollment ON Student.Std_ID = Enrollment.Std_ID
GROUP BY Student.Std_ID, Std_FN, Std_LN