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我确定我可能犯了小错误，但我无法发现错误。我不明白为什么我不能在 MySQL 中使用连接查询。
我在数据库中有三个关系表:
- 类
- 主题
- class_subject
代码是:
CREATE TABLE IF NOT EXISTS `class` (
`ClassID` INT(11) NOT NULL AUTO_INCREMENT ,
`Class` VARCHAR(500) NULL DEFAULT NULL ,
`Section` VARCHAR(500) NULL DEFAULT NULL ,
PRIMARY KEY (`ClassID`))
ENGINE = InnoDB
AUTO_INCREMENT = 15
DEFAULT CHARACTER SET = utf8;
INSERT INTO `class`(`ClassID`, `Class`, `Section`) VALUES ('1','1','A'), ('2','1','B'), ('3','1','C'), ('4','1','D') ;
CREATE TABLE IF NOT EXISTS `Subject` (
`SubjectID` INT(11) NOT NULL AUTO_INCREMENT ,
`Subject` VARCHAR(45) NULL,
PRIMARY KEY (`SubjectID`))
ENGINE = InnoDB
AUTO_INCREMENT = 53
DEFAULT CHARACTER SET = utf8;
INSERT INTO `Subject`(`SubjectID`, `Subject`) VALUES ('1','English'),('2','Hindi'),('3','Maths'),('4','Science'),('5','Social Science'),('6','Sports'),('7','Physics'), ('8','Chemistry'), ('9','Statistics'), ('10','Computers');
和...
CREATE TABLE IF NOT EXISTS `class_subject` (
ClassID INT (11) NOT NULL,
SubjectID INT (11) NOT NULL,
PRIMARY KEY (ClassID, SubjectID),
INDEX `subject_order_idx` (`ClassID` ASC) ,
CONSTRAINT `class_subject`
FOREIGN KEY (`ClassID` )
REFERENCES `class` (`ClassID` )ON UPDATE CASCADE,
FOREIGN KEY (`SubjectID` )
REFERENCES `subject` (`SubjectID` )ON UPDATE CASCADE)
ENGINE = InnoDB
AUTO_INCREMENT = 15
DEFAULT CHARACTER SET = utf8;
INSERT INTO `class_subject`(`ClassID`, `SubjectID`) VALUES ('1','1'), ('1','2'), ('1','3'), ('12','4'), ('12','5'), ('12','6');
加入查询:
$ClassID = "1";
$sql = "SELECT subject.SubjectID, subject.Subject FROM subject " .
"LEFT JOIN class_subject" .
"ON subject.SubjectID = class_subject.SubjectID " .
"WHERE class_subject.ClassID = $ClassID " .
"ORDER BY SubjectID ASC";
尽管相同的查询在 phpMyAdmin 中运行良好。