SQL：内部联接帮助进行评估

Question

在学校的受控评估中，我遇到了以下问题：

创建，运行，测试，解释和演示脚本以执行以下操作：

1. 生成带有OCR考试委员会的所有条目的列表，显示：
   • 带条目的学生姓名
   • 学生参加的考试的科目名称和入学水平。
2. 生成所有学生的列表，显示学生的姓名，然后是要参加的考试。 此列表应按学生姓氏的字母顺序显示。

在我的代码中，我不知道如何使用INNER JOIN联接两个以上的表，但是如果我尝试对'ON'语句不起作用，则不知道如何解决此问题。

CREATE表和INSERT数据：

CREATE TABLE IF NOT EXISTS students
(
student_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
first_name VARCHAR(20) NOT NULL,
middle_name VARCHAR(20),
last_name VARCHAR(40) NOT NULL,
email VARCHAR(60) NOT NULL,
password CHAR(40) NOT NULL,
reg_date DATETIME NOT NULL,
PRIMARY KEY (student_id),
UNIQUE (email)
);

INSERT INTO students (first_name,last_name,email,password,reg_date) VALUES 
("ex1","ex1.1","example1@gmail.com","11062001",'2009-12-04 13:25:30'),
("ex2","ex2.2","example2@gmail.com","ex123",'2015-02-12 15:20:45'),
("my name is jeff","21","kid","mynameis21kid@vine.com","yolo",'2014-09-21 14:15:25'),
("Mr.Right","Mr.Calvin","Mr.Hildfiger","Mr.misters@mister.com","mistermaster",'2015-06-04 19:50:35'),
("Bob","Dabuilda","bobthebuilder@fixit.com","BTBCWFI?",'2005-11-12 21:20:55');

CREATE TABLE IF NOT EXISTS subjects
(
subject_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
subject_name VARCHAR(20) NOT NULL,
level_of_entry VARCHAR(5) NOT NULL,
exam_board VARCHAR(10) NOT NULL,
PRIMARY KEY (subject_id),
UNIQUE(subject_id)
);

INSERT INTO subjects (subject_name,level_of_entry,exam_board) VALUES 
("Chemistry","AS","OCR"),
("Biology","GCSE","AQA"),
("Music","GCSE","Edexcel"),
("English","A","OCR"),
("Physics","A","AQA"),
("Computing","GCSE","Edexcel"),
("French","A","AQA"),
("Maths","AS","OCR"),
("Product Design","GCSE","AQA"),
("History","AS","OCR");

CREATE TABLE IF NOT EXISTS entries
(
entry_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
date_of_exam DATE NOT NULL,
student_id INT UNSIGNED NOT NULL,
subject_id INT UNSIGNED NOT NULL,
FOREIGN KEY (student_id) REFERENCES students(student_id),
FOREIGN KEY (subject_id) REFERENCES subjects(subject_id),
PRIMARY KEY (entry_id)
);

INSERT INTO entries (date_of_exam, student_id, subject_id) VALUES 
('2015-05-31', 1, 6),
('2015-05-31', 2, 10),
('2015-01-21', 3, 3),
('2015-01-21', 4, 7),
('2015-09-13', 5, 1),
('2015-09-13', 2, 9),
('2015-12-06', 4, 8),
('2015-12-06', 1, 2),
('2015-04-01', 3, 5),
('2015-04-01', 5, 4);

和SELECT ：

SELECT entries.*, subjects.subject_name, subjects.level_of_entry
FROM subjects
    INNER JOIN entries,
               students ON entries.subject_id = subjects.subject_id
WHERE subjects.exam_board LIKE "OCR%";

Answer 1

使用显式的JOIN ，立即开始，永不回头！ 还可以使用别名使内容保持整洁。

SELECT e.*, su.subject_name, su.level_of_entry
FROM subjects su
INNER JOIN entries e ON e.subject_id = su.subject_id
INNER JOIN students st ON e.student_id = st.student_id 
WHERE su.exam_board LIKE "OCR%";

Answer 2

在连接之后立即提供连接条件，以使其简单易读。 正如jarlh所写，请勿将显式连接与逗号分隔的列表混合使用。 如果您知道所要查找的确切值，则无需对模式匹配进行匹配。 只需将=运算符与确切值一起使用即可。

SELECT *
FROM subjects
INNER JOIN entries ON entries.subject_id = subjects.subject_id --join 2 tables
INNER JOIN students ON entries.student_id=students.student_id --join the 3rd tables
WHERE subjects.exam_board = "OCR";

Answer 3

我认为您可能需要代码看起来像这样：

select entries.*, subjects.subject_name, subjects.level_of_entry
from entries
join students on students.student_id = entries.student_id
join subjects on subjects.subject_id = entries.subject_id
where exam_board = 'OCR';

我希望这有帮助。