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MySQL 多表联合查询代码示例
MySQL 多表联合查询是一种通过多个表之间的关系,从各个表中提取数据的技术。它允许我们获取跨越多个表的信息,从而进行复杂的数据分析和报告。
语法
SELECT column1, column2, ...
FROM table1
JOIN table2 ON table1.column_name = table2.column_name
[JOIN table3 ON table2.column_name = table3.column_name]
...
[WHERE condition]
[GROUP BY group_column]
[HAVING condition]
[ORDER BY order_column];
示例
示例架构:
+--------------+ +---------------+
| customers | | orders |
+--------------+ +---------------+
| id | | id |
| name | | customer_id |
| address | | order_date |
| phone_number | | total_amount |
+--------------+ +---------------+
查询示例:
获取每个客户的订单总金额:
SELECT c.name AS customer_name, SUM(o.total_amount) AS total_amount
FROM customers AS c
JOIN orders AS o ON c.id = o.customer_id
GROUP BY c.name;
输出:
customer_name | total_amount |
---|---|
John Doe | 1000.00 |
Jane Smith | 500.00 |
其他联合类型
- INNER JOIN:仅返回连接列上匹配的行。
- LEFT JOIN:返回表 1 中的所有行,以及表 2 中与其匹配的行(如果存在)。
- RIGHT JOIN:返回表 2 中的所有行,以及表 1 中与其匹配的行(如果存在)。
- FULL OUTER JOIN:返回表 1 和表 2 中的所有行,无论是否匹配。
性能提示
- 使用索引加快查询速度。
- 优化连接条件,减少不必要的行扫描。
- 避免使用模糊查询(例如
LIKE
和IN
),因为它们会降低性能。 - 考虑使用子查询或视图来将复杂查询分解成更简单的部分。
其他示例
-
获取每个产品类别的销售总额:
SELECT p.category, SUM(o.quantity) AS total_quantity FROM products AS p JOIN order_items AS oi ON p.id = oi.product_id JOIN orders AS o ON oi.order_id = o.id GROUP BY p.category;
-
获取每个客户的最近订单日期:
SELECT c.name AS customer_name, MAX(o.order_date) AS last_order_date FROM customers AS c JOIN orders AS o ON c.id = o.customer_id GROUP BY c.name;
-
获取没有订单的客户:
SELECT c.name AS customer_name FROM customers AS c LEFT JOIN orders AS o ON c.id = o.customer_id WHERE o.id IS NULL;
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