mysql select相加

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    product_price DECIMAL(10, 2),
    quantity INT
);

SELECT order_id, (product_price * quantity) AS total_amount
FROM orders;

-- 错误示例
SELECT order_id, product_price + order_id AS total_amount
FROM orders;

-- 正确示例
SELECT order_id, product_price + CAST(order_id AS DECIMAL(10, 2)) AS total_amount
FROM orders;

-- 错误示例
SELECT product_price + 0.1 AS total_amount
FROM orders;

-- 正确示例
SELECT CAST(product_price AS DECIMAL(10, 2)) + 0.1 AS total_amount
FROM orders;

-- 使用索引优化查询
CREATE INDEX idx_product_price ON orders(product_price);

SELECT order_id, (product_price * quantity) AS total_amount
FROM orders
WHERE product_price > 100;