mysql 怎么合并行

CREATE TABLE students (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    subject VARCHAR(100)
);

INSERT INTO students (id, name, subject) VALUES
(1, 'Alice', 'Math'),
(2, 'Alice', 'Science'),
(3, 'Bob', 'Math'),
(4, 'Bob', 'History');

SELECT name, GROUP_CONCAT(subject SEPARATOR ', ') AS subjects
FROM students
GROUP BY name;

+-------+------------------+
| name  | subjects         |
+-------+------------------+
| Alice | Math, Science    |
| Bob   | Math, History    |
+-------+------------------+

SELECT name, STRING_AGG(subject, ', ' ORDER BY subject) AS subjects
FROM students
GROUP BY name;

+-------+------------------+
| name  | subjects         |
+-------+------------------+
| Alice | Math, Science    |
| Bob   | Math, History    |
+-------+------------------+

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_name VARCHAR(100)
);

CREATE TABLE order_items (
    item_id INT PRIMARY KEY,
    order_id INT,
    product_name VARCHAR(100)
);

INSERT INTO orders (order_id, customer_name) VALUES
(1, 'Alice'),
(2, 'Bob');

INSERT INTO order_items (item_id, order_id, product_name) VALUES
(1, 1, 'Laptop'),
(2, 1, 'Mouse'),
(3, 2, 'Keyboard');

SELECT o.order_id, o.customer_name, GROUP_CONCAT(oi.product_name SEPARATOR ', ') AS products
FROM orders o
JOIN order_items oi ON o.order_id = oi.order_id
GROUP BY o.order_id, o.customer_name;

+----------+-------------+------------------+
| order_id | customer_name | products         |
+----------+-------------+------------------+
|        1 | Alice       | Laptop, Mouse    |
|        2 | Bob         | Keyboard         |
+----------+-------------+------------------+