mysql 定义联合主键

CREATE TABLE students (
    student_id INT,
    course_id INT,
    grade FLOAT,
    PRIMARY KEY (student_id, course_id)
);

CREATE TABLE students (
    student_id INT NOT NULL,
    course_id INT NOT NULL,
    grade FLOAT,
    PRIMARY KEY (student_id, course_id)
);

ALTER TABLE students DROP PRIMARY KEY;

-- 创建表并定义联合主键
CREATE TABLE students (
    student_id INT NOT NULL,
    course_id INT NOT NULL,
    grade FLOAT,
    PRIMARY KEY (student_id, course_id)
);

-- 插入数据
INSERT INTO students (student_id, course_id, grade) VALUES (1, 101, 85.5);
INSERT INTO students (student_id, course_id, grade) VALUES (1, 102, 90.0);
INSERT INTO students (student_id, course_id, grade) VALUES (2, 101, 78.5);

-- 查询数据
SELECT * FROM students WHERE student_id = 1 AND course_id = 101;