Question

学生ID = 4，课程ID = 20没有值，如何显示选择查询？预期输出为一行，其中包含学生详细信息这是我的SQL代码：

create table Students(
    student_id int IDENTITY(1,1) NOT NULL PRIMARY KEY,
    lname varchar(30) NOT NULL,
    fname varchar(30) NOT NULL,
    age int NULL CHECK (age > 18),
    phone char(13) NULL CHECK (phone LIKE '[(][0-9][0-9][0-9][)][0-9][0-9][0-9][-][0-9][0-9][0-9][0-9]')
);

-创建课程表：

create table Course(
    course_id int IDENTITY(10,10) NOT NULL PRIMARY KEY,
    title varchar(50) NULL,
    nb_hrs int NULL
);

-创建成绩表：

create table Grades(
    course_id int NOT NULL FOREIGN KEY REFERENCES Course(course_id),
    student_id int NOT NULL FOREIGN KEY REFERENCES Students(student_id),
    grade float NOT NULL CHECK (grade BETWEEN 0 AND 100)
);

-将数据插入表中

INSERT INTO students(lname, fname, age) VALUES ('Salard', 'Jean', 19)
INSERT INTO students(lname, fname, age) VALUES ('Le Bert', 'Antoine', 24)
INSERT INTO students(lname, fname, age) VALUES ('Legendre', 'Pierre', 23)
INSERT INTO students(lname, fname, age) VALUES ('Turcotte', 'Cedric', 24)
INSERT INTO students(lname, fname, age) VALUES ('Tremblay', 'Alfredo', 22)
INSERT INTO students(lname, fname, age) VALUES ('Dominguez', 'Silvia', 21)
INSERT INTO students(lname, fname, age) VALUES ('Wright', 'Michelle', 30)
INSERT INTO students(lname, fname, age) VALUES ('Sullivan', 'Anne', 20)
INSERT INTO students(lname, fname, age) VALUES ('Petit-Clair', 'Cyril', 21)
INSERT INTO students(lname, fname, age) VALUES ('Herbert', 'Jean-Marc', 32)

INSERT INTO course(title, nb_hrs) VALUES ('P41', 45)
INSERT INTO course(title, nb_hrs) VALUES ('P42', 60)
INSERT INTO course(title, nb_hrs) VALUES ('P60', 60)
INSERT INTO course(title, nb_hrs) VALUES ('P61', 60)

INSERT INTO grades(course_id, student_id, grade) VALUES (10, 1, 78)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 1, 85)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 2, 80)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 2, 90)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 1, 84)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 10, 96)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 2, 61)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 9, 89)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 10, 73)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 3, 45)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 5, 80)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 6, 33)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 4, 67)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 3, 84)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 5, 35)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 5, 85)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 7, 80)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 3, 67)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 6, 77)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 7, 80)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 4, 33)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 6, 56)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 7, 87)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 9, 89)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 10, 73)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 1, 89)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 9, 18)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 2, 50)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 5, 79)
INSERT INTO grades(course_id, student_id, grade) VALUES (30, 8, 37)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 3, 67)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 4, 77)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 8, 75)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 7, 56)
INSERT INTO grades(course_id, student_id, grade) VALUES (40, 8, 87)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 8, 38)
INSERT INTO grades(course_id, student_id, grade) VALUES (20, 6, 92)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 9, 79)
INSERT INTO grades(course_id, student_id, grade) VALUES (10, 10, 97)

（这是一些额外的文本，只是为了我能够在...中编辑代码栏）

Answer 1

最终答案：

选择s.student_id，s.lname，s.fname，s.age，s.phone 来自学生的哪里s.student_id（从g级成绩组中选择student_id，按count（student_id）<4的student_id选择）

感谢所有人

Answer 2

select t1.* from students t1 left outer join grade t2 on t1.student_id=t2.student_id and t2.course_id=20
where t1.student_id=4

需要选择查询以查找空值

2 个答案: