mysql 结果连接

-- 创建 users 表
CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(50)
);

-- 创建 orders 表
CREATE TABLE orders (
    id INT PRIMARY KEY,
    user_id INT,
    amount DECIMAL(10, 2)
);

-- 插入示例数据
INSERT INTO users (id, name) VALUES (1, 'Alice'), (2, 'Bob');
INSERT INTO orders (id, user_id, amount) VALUES (1, 1, 100.00), (2, 1, 200.00), (3, 2, 50.00);

-- 内连接查询
SELECT users.name, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id;

-- 错误的连接条件
SELECT users.name, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id + 1; -- 错误的连接条件

-- 正确的连接条件
SELECT users.name, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id; -- 正确的连接条件

-- 创建索引
CREATE INDEX idx_user_id ON orders(user_id);

-- 优化后的查询语句
SELECT users.name, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id;