MySQL 查询 : display all ungraded essays

Question

到目前为止，我可以列出所有学生的论文和已评分的学生论文，我想知道如何才能显示所有未评分的论文？我还必须向我的 MySQL 代码添加什么？

这是我目前的代码。

SELECT students.*, students_essays.*  
FROM students  
INNER JOIN students_essays ON students.student_id = students_essays.student_id 
INNER JOIN essays_grades ON students_essays.id = essays_grades.students_essays_id
ORDER BY students_essays.id DESC 

这是我的 MySQL 表。

CREATE TABLE students_essays (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
student_id INT UNSIGNED NOT NULL,
content TEXT NOT NULL,
PRIMARY KEY (id)
);


CREATE TABLE students (
student_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
student_first_name VARCHAR(255) DEFAULT NULL,
student_last_name VARCHAR(255) DEFAULT NULL,
pass CHAR(40) NOT NULL,
PRIMARY KEY (student_id)
);


CREATE TABLE essays_grades (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
grade_id INT UNSIGNED NOT NULL,
students_essays_id INT UNSIGNED NOT NULL,
student_id INT UNSIGNED NOT NULL,
PRIMARY KEY (id)
);

CREATE TABLE grades (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
letter_grade VARCHAR(2) DEFAULT NULL,
grade_points FLOAT UNSIGNED NOT NULL DEFAULT 0,
PRIMARY KEY (id)
);

Answer 1

SELECT students.*, students_essays.*  
FROM students  
INNER JOIN students_essays ON students.student_id = students_essays.student_id 
INNER JOIN essays_grades ON students_essays.id = essays_grades.students_essays_id
INNER JOIN grades ON essays_grades.grade_id = grades.id
WHERE LETTER_GRADE IS NULL
ORDER BY students_essays.id DESC;

我使用了 INNER JOIN，就像您所做的一样。请注意，如果在右表中未找到匹配条目，则内部联接将隐藏左表中的行。如果您还想包括这些，请使用左连接。

如果文章未评分，我假设 letter_grade 为空。如果你的数据库没有 essays_grades 行，如果一篇文章没有评分，查询会更简单:

SELECT students.*, students_essays.*  
FROM students  
INNER JOIN students_essays ON students.student_id = students_essays.student_id 
LEFT JOIN essays_grades ON students_essays.id = essays_grades.students_essays_id
WHERE essays_grades.grade_id IS NULL
ORDER BY students_essays.id DESC;