MySQL 窗口函数深度解析：语法、应用场景与性能优化

一、窗口函数核心概念

​​本质​ ​：对一组与当前行相关联的行执行计算，​​不改变原表行数​ ​

​​与聚合函数的区别​​：

SELECT department, AVG(salary)  -- 普通聚合：每个部门一行
FROM employees
GROUP BY department;

SELECT name, salary, 
       AVG(salary) OVER(PARTITION BY department)  -- 窗口函数：保留所有行
FROM employees;

二、窗口函数完整语法结构

函数名([参数]) OVER (
    [PARTITION BY 分组字段]
    [ORDER BY 排序字段 [ASC|DESC]]
    [frame_clause]
)

1. 核心子句详解

• ​PARTITION BY​ ：横向切分窗口，类似 GROUP BY 但不聚合
• ​ORDER BY​：定义窗口内排序，影响排名类函数和累计计算
• ​frame_clause​：指定窗口范围，语法：

ROWS | RANGE BETWEEN frame_start AND frame_end

2. 窗口帧类型（frame_clause）

类型	示例	说明
默认帧	RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW	默认与排序字段相关
物理行偏移	ROWS BETWEEN 2 PRECEDING AND 1 FOLLOWING	包含前2行到后1行
逻辑值范围	RANGE BETWEEN INTERVAL 7 DAY PRECEDING AND CURRENT ROW	时间范围窗口

三、常用窗口函数分类

1. 排序函数

SELECT 
    product_id,
    sales,
    ROW_NUMBER() OVER(ORDER BY sales DESC) AS rank1,  -- 唯一连续序号
    RANK() OVER(ORDER BY sales DESC) AS rank2,        -- 允许并列跳号
    DENSE_RANK() OVER(ORDER BY sales DESC) AS rank3   -- 允许并列不跳号
FROM sales_data;

2. 聚合窗口函数

SELECT 
    date,
    revenue,
    SUM(revenue) OVER(ORDER BY date 
                      ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS 3d_avg,
    AVG(revenue) OVER(PARTITION BY YEAR(date)) AS year_avg
FROM daily_sales;

3. 分布分析函数

SELECT 
    student_id,
    score,
    PERCENT_RANK() OVER(ORDER BY score) AS percentile,  -- 相对百分比排名
    CUME_DIST() OVER(ORDER BY score) AS cumulative_dist -- 累计分布
FROM exam_results;

4. 偏移函数

SELECT 
    date,
    temperature,
    LAG(temperature, 1) OVER(ORDER BY date) AS prev_day_temp,  -- 前一行
    LEAD(temperature, 1) OVER(ORDER BY date) AS next_day_temp, -- 后一行
    FIRST_VALUE(temperature) OVER(PARTITION BY WEEK(date)) AS week_first_temp
FROM weather;

四、典型应用场景

1. 动态分组TopN

-- 获取每个部门薪资前三名
WITH ranked AS (
    SELECT 
        name, department, salary,
        DENSE_RANK() OVER(PARTITION BY department ORDER BY salary DESC) AS rk
    FROM employees
)
SELECT * FROM ranked WHERE rk <= 3;

2. 累计计算

-- 计算累计销售额与移动平均
SELECT 
    order_date,
    daily_sales,
    SUM(daily_sales) OVER(ORDER BY order_date) AS cum_sum,
    AVG(daily_sales) OVER(ORDER BY order_date 
                          ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) AS 7d_ma
FROM orders;

3. 数据差异分析

-- 对比每月销售额与上月差异
SELECT 
    month,
    revenue,
    LAG(revenue, 1) OVER(ORDER BY month) AS prev_month,
    revenue - LAG(revenue, 1) OVER(ORDER BY month) AS mom_diff
FROM monthly_sales;

五、性能优化策略

1. 索引设计

-- 为窗口函数涉及的字段建立组合索引
ALTER TABLE sales ADD INDEX idx_dept_time (department, order_date);

-- 执行计划检查
EXPLAIN 
SELECT 
    product_id,
    SUM(quantity) OVER(PARTITION BY product_id ORDER BY sale_date)
FROM sales;

2. 分区剪枝优化

-- 结合WHERE条件减少处理分区
SELECT *
FROM (
    SELECT 
        user_id,
        ROW_NUMBER() OVER(PARTITION BY city ORDER BY reg_date) AS rn
    FROM users
    WHERE city IN ('北京','上海')  -- 提前过滤
) t
WHERE rn <= 100;

3. 避免重复计算

-- 使用WINDOW子句复用定义
SELECT 
    AVG(sales) OVER w AS avg_sales,
    MAX(sales) OVER w AS max_sales
FROM sales_data
WINDOW w AS (PARTITION BY region ORDER BY month);

六、与临时表结合的高级用法

1. 分阶段计算

-- 第一阶段：计算基础窗口
CREATE TEMPORARY TABLE stage1 AS
SELECT 
    user_id,
    SUM(amount) OVER(PARTITION BY user_id) AS total_amt
FROM transactions;

-- 第二阶段：二次聚合
SELECT 
    AVG(total_amt) AS avg_amt_per_user
FROM stage1;

2. 递归窗口计算

-- 计算员工管理链层级
WITH RECURSIVE emp_tree AS (
    SELECT 
        emp_id, 
        manager_id, 
        1 AS level,
        CAST(emp_id AS CHAR(100)) AS path
    FROM employees
    WHERE manager_id IS NULL
    
    UNION ALL
    
    SELECT 
        e.emp_id,
        e.manager_id,
        et.level + 1,
        CONCAT(et.path, '->', e.emp_id)
    FROM employees e
    JOIN emp_tree et ON e.manager_id = et.emp_id
)
SELECT 
    emp_id,
    level,
    path,
    RANK() OVER(ORDER BY level) AS hierarchy_rank
FROM emp_tree;

七、常见错误排查

​​错误现象​​	​​原因分析​​	​​解决方案​​
结果排序不符合预期	未正确使用ORDER BY	明确指定排序字段和方向
窗口范围计算错误	ROWS与RANGE混淆使用	确认需要物理行偏移还是逻辑值范围
性能急剧下降	未加分区条件导致全表扫描	添加WHERE条件或分区过滤
出现重复排名	使用了ROW_NUMBER而非RANK	根据业务需求选择正确的排名函数

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八、版本特性差异

​​MySQL版本​​	​​窗口函数支持​​
5.x	❌ 不支持
8.0.2+	✔️ 基础窗口函数
8.0.28+	✔️ 新增NTH_VALUE()、NTILE()等扩展函数

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结语：最佳实践原则

1. ​明确窗口范围​ ：始终指定ROWS/RANGE避免意外结果

2. ​慎用无界窗口​ ：UNBOUNDED PRECEDING可能导致性能问题

3. ​结合索引优化​ ：为PARTITION BY和ORDER BY字段建立索引

4. ​分阶段处理​：对复杂计算使用临时表拆分步骤

5. ​版本验证​ ：生产环境确认MySQL版本支持情况

   -- 查看窗口函数执行计划
   EXPLAIN FORMAT=JSON
   SELECT ... [包含窗口函数的查询];
   
   -- 性能模式监控
   SELECT * FROM performance_schema.events_stages_history_long
   WHERE EVENT_NAME LIKE '%window%';