如何關聯兩個沒有關聯表的表？

Question

它目前我正在使用。在那我想減少關聯表。並達到相同的目標。一些人幫助我找到解決方案。

CREATE TABLE sys_student_t (
student_id MEDIUMINT NOT NULL AUTO_INCREMENT, 
student_name VARCHAR(255), 
PRIMARY KEY (student_id) 
); 

CREATE TABLE sys_classrooms_t (
    classroom_id MEDIUMINT NOT NULL AUTO_INCREMENT, 
    classroom_name VARCHAR(255), 
    PRIMARY KEY (classroom_id) 
); 

CREATE TABLE student_classroom_association_t (
    association_id MEDIUMINT NOT NULL AUTO_INCREMENT, 
    classroom_id MEDIUMINT NOT NULL, 
    student_id MEDIUMINT NOT NULL, 
    FOREIGN KEY (student_id) REFERENCES sys_student_t (student_id), 
    FOREIGN KEY (classroom_id) REFERENCES sys_classrooms_t (classroom_id), 
    PRIMARY KEY (association_id) 
); 

這裏是小提琴鏈接： - http://www.sqlfiddle.com/#!9/ada4fd/2 感謝

Answer 1

A「許多一對多」的關係總是需要一個額外的表。您唯一能做的就是將關聯表的主鍵更改爲student_id和classroom_id。

CREATE TABLE student_classroom_association_t (
    classroom_id MEDIUMINT NOT NULL, 
    student_id MEDIUMINT NOT NULL, 
    FOREIGN KEY (student_id) REFERENCES sys_student_t (student_id), 
    FOREIGN KEY (classroom_id) REFERENCES sys_classrooms_t (classroom_id), 
    PRIMARY KEY (student_id, classroom_id) 
);