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CTE 的主要优势_以MySQL为例

news 2025/9/3 0:10:30

CTE 的主要优势

1. 提高可读性和可维护性

优势说明：将复杂的查询分解成多个逻辑部分，每个部分都有一个有意义的名称。

传统子查询方式（难以阅读）：

sql

SELECT *
FROM (SELECT customer_id, SUM(amount) as total_spentFROM orders WHERE order_date >= '2023-01-01'GROUP BY customer_id
) AS subquery
WHERE total_spent > 1000;

使用 CTE 方式（清晰易读）：

sql

WITH customer_totals AS (SELECT customer_id, SUM(amount) as total_spentFROM orders WHERE order_date >= '2023-01-01'GROUP BY customer_id
)
SELECT *
FROM customer_totals
WHERE total_spent > 1000;

2. 避免重复代码

优势说明：当同一个子查询需要在多个地方使用时，CTE 只需定义一次，可多次引用。

示例：

sql

WITH high_value_orders AS (SELECT * FROM orders WHERE amount > 1000
)
SELECT (SELECT COUNT(*) FROM high_value_orders) as total_count,(SELECT AVG(amount) FROM high_value_orders) as average_amount,hv.*
FROM high_value_orders hv
WHERE hv.customer_id IN (SELECT customer_id FROM high_value_orders GROUP BY customer_id HAVING COUNT(*) > 5
);

3. 支持递归查询

优势说明：CTE 支持递归，可以处理树形结构或层次化数据。

示例：查询组织架构中的所有下属

sql

WITH RECURSIVE org_chart AS (-- 锚点成员：找到指定员工SELECT employee_id, name, manager_id, 1 as levelFROM employeesWHERE employee_id = 101UNION ALL-- 递归成员：找到下属SELECT e.employee_id, e.name, e.manager_id, oc.level + 1FROM employees eINNER JOIN org_chart oc ON e.manager_id = oc.employee_id
)
SELECT * FROM org_chart;

4. 替代视图的临时使用

优势说明：当需要临时创建一个"视图"但不想永久保存在数据库中时，CTE 是完美选择。

示例：

sql

-- 不需要创建永久视图，只需在查询中临时定义
WITH monthly_sales AS (SELECT YEAR(order_date) as year,MONTH(order_date) as month,SUM(amount) as total_sales,COUNT(*) as order_countFROM ordersGROUP BY YEAR(order_date), MONTH(order_date)
)
SELECT year, month,total_sales,order_count,total_sales / order_count as avg_order_value
FROM monthly_sales
ORDER BY year, month;

5. 简化复杂查询的调试过程

优势说明：可以逐步构建和测试复杂查询的各个部分。

示例：逐步构建销售分析报告

sql

-- 第一步：先测试基础CTE
WITH customer_orders AS (SELECT customer_id, COUNT(*) as order_count, SUM(amount) as total_spentFROM ordersGROUP BY customer_id
)
SELECT * FROM customer_orders; -- 先测试这部分-- 第二步：添加更多CTE和逻辑
WITH customer_orders AS (SELECT customer_id, COUNT(*) as order_count, SUM(amount) as total_spentFROM ordersGROUP BY customer_id
),
active_customers AS (SELECT * FROM customer_orders WHERE order_count >= 3
)
SELECT c.name,ac.order_count,ac.total_spent,ac.total_spent / ac.order_count as avg_order_value
FROM active_customers ac
JOIN customers c ON ac.customer_id = c.id;

6. 更好的性能优化（在某些情况下）

优势说明：MySQL 优化器有时可以更好地优化 CTE，尤其是当 CTE 被多次引用时。

示例：

sql

WITH product_stats AS (SELECT product_id,AVG(quantity) as avg_quantity,MAX(price) as max_priceFROM order_detailsGROUP BY product_id
)
SELECT p.product_name,ps.avg_quantity,ps.max_price,(SELECT COUNT(*) FROM orders o JOIN order_details od ON o.order_id = od.order_id WHERE od.product_id = p.product_id) as total_orders
FROM products p
JOIN product_stats ps ON p.product_id = ps.product_id;

7. 支持多个 CTE 的链式使用

优势说明：可以在一个查询中定义多个 CTE，每个都可以基于前一个的结果。

示例：

sql

WITH 
-- 第一个CTE：计算订单总数
order_counts AS (SELECT customer_id, COUNT(*) as total_ordersFROM ordersGROUP BY customer_id
),
-- 第二个CTE：筛选高价值客户
high_value_customers AS (SELECT customer_id, total_ordersFROM order_countsWHERE total_orders > 10
),
-- 第三个CTE：获取客户详情
customer_details AS (SELECT c.*, hvc.total_ordersFROM customers cJOIN high_value_customers hvc ON c.customer_id = hvc.customer_id
)
-- 主查询
SELECT customer_id,name,email,total_orders,CASE WHEN total_orders > 20 THEN '钻石客户'WHEN total_orders > 15 THEN '黄金客户'ELSE '白银客户'END as customer_level
FROM customer_details
ORDER BY total_orders DESC;