MySQL查詢問題

Question

到目前爲止，我可以列出所有的學生論文和分級學生論文，並想知道我怎麼會能夠顯示所有未分級的文章？我還有什麼要添加到我的MySQL代碼？

感謝您的幫助！

這是我到目前爲止的代碼。

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 

這是我的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; 

我用INNER JOIN，就像你一樣。需要注意的是內部聯接將隱藏行的左側表中，如果右表中沒有找到匹配條目。如果您還想包含這些，請使用左連接。

我認爲letter_grade爲空，如果作文是不分級的。如果你的數據庫不會有essays_grades行，如果文章沒有分級，查詢會更簡單：

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; 

Answer 2

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.students_essays_id is null 
ORDER BY students_essays.id DESC 

Answer 3

是不分級的一個letter_grade，E。 G。 'UG'？

在這種情況下，ORDER BY前加

WHERE letter_grade = 'UG' 

。