SQLite Join查询返回不存在的数据

Question

我正在SQLite中的一个简单的Student / teacher数据库上执行join查询。 奇怪的是，以下查询返回的数据产生的结果以前未插入表中。

SELECT ss.crsName, ss.stdName, ts.teacherName, ts.term from TeacherSchedule as ts 
inner join StudentSchedule as ss on ss.crsName = ts.crsName 
inner join Teacher as t on t.teacherName = ts.teacherName 
where ss.crsName like 'COMP4900'
order by ss.stdName;

例如，在上图中，您可以看到学生JC从COMP4900和tej中提取了farnaz，但是下面的insert语句实际上从未指定JC与COMP4900一起使用farnaz，但仅与tej一起使用。

DROP TABLE Student;
DROP TABLE Course;
drop table Teacher;
drop table StudentSchedule; 
drop table TeacherSchedule;

PRAGMA foreign_keys = on;

CREATE TABLE Student (
  stdID INTEGER PRIMARY KEY  AUTOINCREMENT,
  stdName   TEXT NOT NULL,
  stdAge        INT NOT NULL,
  stdAddress    VARCHAR(50) NOT NULL
);

CREATE TABLE Teacher (
  teacherID INTEGER PRIMARY KEY  AUTOINCREMENT,
  teacherName   TEXT NOT NULL,
  teacherAge        INT NOT NULL,
  teacherAddress    VARCHAR(50) NOT NULL
);

create table Course (
  crsID INTEGER PRIMARY KEY  AUTOINCREMENT,
  crsName VARCHAR(8) NOT NULL,
  crsTerm INT NOT NULL
);

CREATE TABLE StudentSchedule(
  stdSchedule INTEGER PRIMARY KEY  AUTOINCREMENT,
  stdName INT NOT NULL,
  teacherName INT not NULL,
  crsName INT not NULL,
  startTime NUMERIC NOT NULL,
  endTime NUMERIC NOT NULL,
  term INT NOT NULL,
  FOREIGN KEY(stdName) references Student(stdID),
  FOREIGN KEY(teacherName) REFERENCES Teacher(teacherID),
  FOREIGN KEY(crsName) REFERENCES Course(crsID)
);

CREATE TABLE TeacherSchedule(
  teacherSchedule INTEGER PRIMARY KEY  AUTOINCREMENT,
  teacherName INT not NULL,
  crsName INT not NULL,
  startTime NUMERIC NOT NULL,
  endTime NUMERIC NOT NULL,
  term INT NOT NULL,
  FOREIGN KEY(teacherName) REFERENCES Teacher(teacherID),
  FOREIGN KEY(crsName) REFERENCES Course(crsID)
);


insert into Student (stdName, stdAge, stdAddress) values ('John', 25, 'vancouver, bc');
insert into Student (stdName, stdAge, stdAddress) values ('Rost', 25, 'Delta, bc');
insert into Student (stdName, stdAge, stdAddress) values ('Mike', 29, 'vancouver, bc');
insert into Student (stdName, stdAge, stdAddress) values ('Johnny', 27, 'surrey, bc');
insert into Student (stdName, stdAge, stdAddress) values ('Jeff', 27, 'vancouver, bc');
insert into Student (stdName, stdAge, stdAddress) values ('Li', 32, 'vancouver, bc');
insert into Student (stdName, stdAge, stdAddress) values ('JC', 24, 'richmond, bc');

Insert into Teacher(teacherName, teacherAge, teacherAddress) values ('tej', 65, 'surrey');
Insert into Teacher(teacherName, teacherAge, teacherAddress) values ('aman', 70, 'surrey');
Insert into Teacher(teacherName, teacherAge, teacherAddress) values ('chris', 40, 'vancouver');
Insert into Teacher(teacherName, teacherAge, teacherAddress) values ('farnaz', 41, 'delta');
Insert into Teacher(teacherName, teacherAge, teacherAddress) values ('richard', 99, 'richmond');

insert into Course (crsName, crsTerm) values ('COMP1510', 1);
insert into Course (crsName, crsTerm) values ('COMP2510', 2);
insert into Course (crsName, crsTerm) values ('COMP4100', 3);
insert into Course (crsName, crsTerm) values ('COMP4900', 3);
insert into Course (crsName, crsTerm) values ('COMP2546', 2);
insert into Course (crsName, crsTerm) values ('COMP1536', 1);

insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('Johnny','tej','COMP1510',1200,200,1);
insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('Mike','farnaz','COMP2526',400,600,3);
insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('JC','richard','COMP1536',1200,400,1);

insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('Matt','tej','COMP4900',1000,1200,2);
insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('Jeff','tej','COMP4900',1000,1200,2);
insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('JC','tej','COMP4900',1000,1200,2);

insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('Rost','farnaz','COMP4900',1200,200,3);
insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('Jeff','farnaz','COMP4900',1200,200,3);
insert into StudentSchedule(stdName, teacherName, crsName, startTime, endTime, term) values ('John','farnaz','COMP4900',1200,200,3);

insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('tej', 'COMP1510', 1200, 200, 1);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('tej', 'COMP4900', 1000, 200, 2);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('tej', 'COMP4100', 1000, 1200, 1);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('aman', 'COMP2510', 1200, 200, 1);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('aman', 'COMP1510', 300, 400, 1);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('chris', 'COMP4100', 200, 400, 2);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('farnaz', 'COMP2526', 400, 600, 3);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('richard', 'COMP1536', 1200, 400, 1);
insert into TeacherSchedule(teacherName, crsName, startTime, endTime, term) values ('farnaz', 'COMP4900', 1200, 200, 3);

有人可以解释这是为什么还是看到问题？

Answer 1

在查询中，您忘记了指定Teacher.teacherName应该与StudentSchedule.teacherName匹配。您正在加入学生和老师不匹配的行，从而创建了多余的行。

此查询将为您提供所需的行。

SELECT ss.crsName, ss.stdName, ts.teacherName, ts.term from TeacherSchedule as ts 
inner join StudentSchedule as ss on ss.crsName = ts.crsName
inner join Teacher as t on t.teacherName = ts.teacherName AND t.teacherName = ss.teacherName
where ss.crsName like 'COMP4900'
order by ss.stdName;