mysql 主表和子表

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE
);

CREATE TABLE order_items (
    item_id INT PRIMARY KEY,
    order_id INT,
    product_id INT,
    quantity INT,
    FOREIGN KEY (order_id) REFERENCES orders(order_id)
);

-- 示例：优化查询
SELECT o.order_id, o.customer_id, o.order_date, oi.product_id, oi.quantity
FROM orders o
JOIN order_items oi ON o.order_id = oi.order_id
WHERE o.order_date BETWEEN '2023-01-01' AND '2023-12-31';

CREATE TABLE students (
    student_id INT PRIMARY KEY,
    student_name VARCHAR(100)
);

CREATE TABLE courses (
    course_id INT PRIMARY KEY,
    course_name VARCHAR(100)
);

CREATE TABLE student_courses (
    student_id INT,
    course_id INT,
    PRIMARY KEY (student_id, course_id),
    FOREIGN KEY (student_id) REFERENCES students(student_id),
    FOREIGN KEY (course_id) REFERENCES courses(course_id)
);