mysql联合主键的特点

INSERT INTO table_name (column1, column2, column3)
SELECT * FROM (SELECT 'value1' AS column1, 'value2' AS column2, 'value3' AS column3) AS tmp
WHERE NOT EXISTS (
    SELECT column1, column2 FROM table_name WHERE column1 = 'value1' AND column2 = 'value2'
) LIMIT 1;

UPDATE table_name
SET column1 = 'new_value1', column2 = 'new_value2'
WHERE column1 = 'old_value1' AND column2 = 'old_value2';

-- 创建包含联合主键的表
CREATE TABLE students_courses (
    student_id INT NOT NULL,
    course_id INT NOT NULL,
    grade CHAR(1),
    PRIMARY KEY (student_id, course_id)
);

-- 插入数据
INSERT INTO students_courses (student_id, course_id, grade) VALUES (1, 101, 'A');
INSERT INTO students_courses (student_id, course_id, grade) VALUES (1, 102, 'B');
INSERT INTO students_courses (student_id, course_id, grade) VALUES (2, 101, 'C');

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