Question

我正在为即将到来的数据库考试做练习，并且试图使嵌套和SQL中的多个联接（尤其是Postgresql语法）更加深入人心。我想返回所有获得A级成绩的学生的所有学生姓名和部门名称。

这是我的模式。

CREATE TABLE student1 (
    student_number INTEGER NOT NULL PRIMARY KEY,
    name TEXT NOT NULL,
    class INTEGER NOT NULL,
    major TEXT NOT NULL
);


CREATE TABLE course1 (
    course_name TEXT NOT NULL,
    course_number TEXT NOT NULL PRIMARY KEY,
    credit_hours INTEGER NOT NULL,
    department TEXT NOT NULL
);

CREATE TABLE section1 (
    section_identifer INTEGER NOT NULL PRIMARY KEY,
    course_number TEXT NOT NULL,
    semester TEXT NOT NULL,
    year INTEGER NOT NULL,
    instructor TEXT NOT NULL,
    FOREIGN KEY (course_number) REFERENCES course1(course_number) ON DELETE CASCADE
);

CREATE TABLE grade_report1 (
    id SERIAL NOT NULL PRIMARY KEY,
    student_number INTEGER NOT NULL,
    section_identifer INTEGER NOT NULL,
    grade TEXT NOT NULL,
    FOREIGN KEY (student_number) REFERENCES student1(student_number) ON DELETE CASCADE,
    FOREIGN KEY (section_identifer) REFERENCES section1(section_identifer) ON DELETE CASCADE
);

我整理了一个我认为行得通的嵌套语句：

SELECT t1.name, t3.department
FROM (student1 t1 INNER JOIN grade_report1 t2 ON t1.student_number = t2.student_number) t5
INNER JOIN (course1 t3 INNER JOIN section1 t4 ON t3.course_number = t4.course_number) t6
ON t5.section_identifer = t6.section_identifer
WHERE t2.grade = 'A';

但是，这给了我错误invalid reference to FROM-clause entry for table "t1"。我猜是因为那不是您应该如何命名/引用JOINS的方式。我想要一种将所有这些表联接在一起的方法。谢谢！

Answer 1

删除括号并修复别名：

SELECT s.name, c.department
FROM student1 s INNER JOIN
     grade_report1 gr
     ON gr.student_number = s.student_number INNER JOIN
     section1 sec
     ON sec.section_identifer = gr.section_identifer INNER JOIN
     course1 c 
     ON sec.course_number = c.course_number
WHERE gr.grade = 'A';

允许使用括号，但不需要。使用括号（非常非常需要）时，它们不会获得单独的别名。

在Postgresql中联接多个表

1 个答案: