mysql数据库聚合函数

CREATE TABLE sales (
    id INT AUTO_INCREMENT PRIMARY KEY,
    product_name VARCHAR(255),
    quantity INT,
    sale_date DATE
);

SELECT SUM(quantity) AS total_quantity FROM sales;

SELECT AVG(quantity) AS avg_quantity FROM sales;

SELECT product_name, SUM(quantity) AS total_quantity FROM sales GROUP BY product_name;

-- 错误示例
SELECT product_name, quantity FROM sales GROUP BY product_name;

-- 正确示例
SELECT product_name, SUM(quantity) AS total_quantity FROM sales GROUP BY product_name;

-- 错误示例
SELECT product_name, SUM(quantity) AS total_quantity FROM sales WHERE SUM(quantity) > 100 GROUP BY product_name;

-- 正确示例
SELECT product_name, SUM(quantity) AS total_quantity FROM sales GROUP BY product_name HAVING total_quantity > 100;