mysql 表嵌套查询语句

CREATE TABLE customers (
    customer_id INT PRIMARY KEY,
    customer_name VARCHAR(100)
);

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE,
    amount DECIMAL(10, 2)
);

SELECT customer_name
FROM customers
WHERE customer_id IN (
    SELECT customer_id
    FROM orders
    WHERE amount > (
        SELECT AVG(amount)
        FROM orders
    )
);

SELECT customer_name, (
    SELECT COUNT(*)
    FROM orders
    WHERE orders.customer_id = customers.customer_id
) AS order_count
FROM customers;

SELECT customer_name
FROM customers
WHERE EXISTS (
    SELECT 1
    FROM orders
    WHERE orders.customer_id = customers.customer_id
);

SELECT c.customer_name
FROM customers c
JOIN orders o ON c.customer_id = o.customer_id
GROUP BY c.customer_name;