MySQL连表查询从基础到实战案例

SELECT 列名
FROM 表1
INNER JOIN 表2 ON 表1.列 = 表2.列;

-- 查询有订单的客户信息
SELECT c.customer_name, o.order_date, o.amount
FROM customers c
INNER JOIN orders o ON c.customer_id = o.customer_id;

SELECT 列名
FROM 表1
LEFT JOIN 表2 ON 表1.列 = 表2.列;

-- 查询所有客户及其订单（包括没有订单的客户）
SELECT c.customer_name, o.order_date, o.amount
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id;

SELECT 列名
FROM 表1
RIGHT JOIN 表2 ON 表1.列 = 表2.列;

-- 查询所有订单及其客户信息（包括没有客户信息的订单）
SELECT c.customer_name, o.order_date, o.amount
FROM customers c
RIGHT JOIN orders o ON c.customer_id = o.customer_id;

SELECT 列名 FROM 表1 LEFT JOIN 表2 ON 条件
UNION
SELECT 列名 FROM 表1 RIGHT JOIN 表2 ON 条件;

SELECT 列名
FROM 表1
CROSS JOIN 表2;

-- 生成所有可能的颜色和尺寸组合
SELECT colors.color_name, sizes.size_value
FROM colors
CROSS JOIN sizes;

SELECT 列名
FROM 表 AS 别名1
JOIN 表 AS 别名2 ON 条件;

-- 查询员工及其经理姓名（假设manager_id引用employee_id）
SELECT e.employee_name, m.employee_name AS manager_name
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.employee_id;

SELECT 列名
FROM 表1
JOIN 表2 ON 条件
JOIN 表3 ON 条件;

-- 查询订单详情，包括客户信息和产品信息
SELECT 
    o.order_id, 
    c.customer_name, 
    p.product_name, 
    od.quantity, 
    od.unit_price
FROM orders o
JOIN customers c ON o.customer_id = c.customer_id
JOIN order_details od ON o.order_id = od.order_id
JOIN products p ON od.product_id = p.product_id;

EXPLAIN SELECT ... [你的连表查询];

-- 统计每个客户的订单总数和总金额
SELECT 
    c.customer_id,
    c.customer_name,
    COUNT(o.order_id) AS total_orders,
    SUM(od.quantity * od.unit_price) AS total_amount
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id
LEFT JOIN order_details od ON o.order_id = od.order_id
GROUP BY c.customer_id, c.customer_name;

-- 查询用户A的好友及其共同好友（假设使用自连接）
SELECT 
    u1.user_name AS user_a,
    u2.user_name AS friend,
    COUNT(u3.user_id) AS mutual_friends_count
FROM friendships f1
JOIN users u1 ON f1.user_id = u1.user_id
JOIN users u2 ON f1.friend_id = u2.user_id
LEFT JOIN friendships f2 ON f1.friend_id = f2.user_id AND f2.friend_id = [用户A的ID]
LEFT JOIN users u3 ON f2.friend_id = u3.user_id
WHERE u1.user_id = [用户A的ID]
GROUP BY u1.user_name, u2.user_name;