# 窗口函数速查

## 基本语法

```sql
函数名() OVER (
    PARTITION BY 分组字段      -- 可选：分组
    ORDER BY 排序字段          -- 可选：排序
    ROWS/RANGE 窗口范围        -- 可选：窗口范围
)
```

---

## 排序函数

| 函数 | 说明 | 特点 | 适用场景 |
|------|------|------|----------|
| ROW_NUMBER() | 连续排名 | 不跳号，相同值不同排名 | 每组取前N条、去重 |
| RANK() | 排名 | 跳号，相同值相同排名 | 成绩排名 |
| DENSE_RANK() | 紧密排名 | 不跳号，相同值相同排名 | 连续名次 |
| NTILE(n) | 分桶 | 分成n组 | 数据分片、抽样 |

### ROW_NUMBER 示例

```sql
-- 每个部门薪资最高的员工（取第一名）
SELECT *
FROM (
    SELECT 
        name,
        department,
        salary,
        ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rn
    FROM employees
) t
WHERE rn = 1

-- 去重：每个用户取最新订单
SELECT *
FROM (
    SELECT 
        *,
        ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn
    FROM orders
) t
WHERE rn = 1
```

### RANK 与 DENSE_RANK 示例

```sql
-- 成绩排名（跳号）
SELECT 
    name,
    score,
    RANK() OVER (ORDER BY score DESC) AS rank,         -- 1,2,2,4,5...
    DENSE_RANK() OVER (ORDER BY score DESC) AS dense   -- 1,2,2,3,4...
FROM students
```

### NTILE 示例

```sql
-- 将用户分成4组（业绩分位）
SELECT 
    name,
    sales,
    NTILE(4) OVER (ORDER BY sales DESC) AS quartile   -- 1(最高)到4(最低)
FROM sales_data
```

---

## 聚合函数（窗口内）

| 函数 | 说明 |
|------|------|
| SUM(col) OVER | 累计求和 |
| AVG(col) OVER | 累计平均 |
| COUNT(col) OVER | 窗口内计数 |
| MAX(col) OVER | 窗口内最大值 |
| MIN(col) OVER | 窗口内最小值 |

### 累计求和示例

```sql
-- 累计销售额（从开始到当前）
SELECT 
    date,
    amount,
    SUM(amount) OVER (ORDER BY date) AS cumulative_amount
FROM daily_sales

-- 分组累计（每个部门累计）
SELECT 
    date,
    department,
    amount,
    SUM(amount) OVER (PARTITION BY department ORDER BY date) AS dept_cumulative
FROM sales_data
```

### 移动平均示例

```sql
-- 7天移动平均
SELECT 
    date,
    amount,
    AVG(amount) OVER (
        ORDER BY date
        ROWS BETWEEN 6 PRECEDING AND CURRENT ROW
    ) AS moving_avg_7d
FROM daily_sales
```

---

## 偏移函数

| 函数 | 说明 | 适用场景 |
|------|------|----------|
| LAG(col, n) | 取前n行的值 | 环比、查看历史值 |
| LAG(col, n, default) | 取前n行，无值返回default | 防止NULL |
| LEAD(col, n) | 取后n行的值 | 查看未来值 |
| LEAD(col, n, default) | 取后n行，无值返回default | 防止NULL |
| FIRST_VALUE(col) | 窗口第一个值 | 组内首个值 |
| LAST_VALUE(col) | 窗口最后一个值 | 组内末尾值 |

### LAG 示例（环比计算）

```sql
-- 日环比增长
SELECT 
    date,
    amount,
    LAG(amount, 1) OVER (ORDER BY date) AS prev_day,
    amount - LAG(amount, 1) OVER (ORDER BY date) AS daily_growth,
    ROUND((amount - LAG(amount, 1) OVER (ORDER BY date)) 
        / LAG(amount, 1) OVER (ORDER BY date) * 100, 2) AS growth_rate_pct
FROM daily_sales

-- 月度同比（取去年同期）
SELECT 
    month,
    revenue,
    LAG(revenue, 12) OVER (ORDER BY month) AS prev_year_revenue,
    ROUND((revenue - LAG(revenue, 12) OVER (ORDER BY month)) 
        / LAG(revenue, 12) OVER (ORDER BY month) * 100, 2) AS yoy_growth_pct
FROM monthly_revenue
```

### LEAD 示例

```sql
-- 查看下一行数据
SELECT 
    date,
    amount,
    LEAD(amount, 1) OVER (ORDER BY date) AS next_day_amount
FROM daily_sales
```

### FIRST_VALUE / LAST_VALUE 示例

```sql
-- 每个部门薪资最高和最低的人
SELECT 
    name,
    department,
    salary,
    FIRST_VALUE(name) OVER (PARTITION BY department ORDER BY salary DESC) AS highest_paid,
    LAST_VALUE(name) OVER (
        PARTITION BY department 
        ORDER BY salary DESC
        ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
    ) AS lowest_paid
FROM employees
```

**注意**：LAST_VALUE 默认窗口是 `ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`，需要显式指定全窗口才能取到最后值。

---

## 窗口范围定义

### ROWS（基于行数）

```sql
ROWS BETWEEN 3 PRECEDING AND CURRENT ROW       -- 前3行到当前行
ROWS BETWEEN 6 PRECEDING AND 1 FOLLOWING       -- 前6行到后1行
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW  -- 从开始到当前
ROWS BETWEEN CURRENT ROW AND 3 FOLLOWING       -- 当前行到后3行
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING  -- 全窗口
ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING       -- 当前行及前后各1行（共3行）
```

### RANGE（基于值范围）

```sql
RANGE BETWEEN 100 PRECEDING AND CURRENT ROW    -- 值差100以内
RANGE BETWEEN INTERVAL '7' DAY PRECEDING AND CURRENT ROW  -- 7天内
```

**ROWS vs RANGE**：
- ROWS：固定行数，不受值影响
- RANGE：根据排序字段的值计算范围

---

## 实战案例

### 案例1：分组取Top N

```sql
-- 每个部门薪资前3名
SELECT *
FROM (
    SELECT 
        name,
        department,
        salary,
        ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rn
    FROM employees
) t
WHERE rn <= 3
```

### 案例2：连续问题判断

```sql
-- 判断是否连续增长（连续3天增长）
SELECT 
    date,
    amount,
    CASE 
        WHEN amount > LAG(amount, 1) OVER (ORDER BY date)
         AND LAG(amount, 1) OVER (ORDER BY date) > LAG(amount, 2) OVER (ORDER BY date)
        THEN '连续增长'
        ELSE '非连续增长'
    END AS trend
FROM daily_sales
```

### 案例3：组内占比

```sql
-- 每个部门各员工薪资占比
SELECT 
    name,
    department,
    salary,
    ROUND(salary / SUM(salary) OVER (PARTITION BY department) * 100, 2) AS salary_pct
FROM employees
```

### 案例4：累计百分比（帕累托分析）

```sql
-- 80/20分析：哪些客户贡献了80%销售额
SELECT 
    customer_id,
    sales_amount,
    ROUND(SUM(sales_amount) OVER (ORDER BY sales_amount DESC) 
        / SUM(sales_amount) OVER () * 100, 2) AS cumulative_pct
FROM customer_sales
ORDER BY sales_amount DESC
```

### 案例5：缺失值填充

```sql
-- 用前一个有效值填充NULL
SELECT 
    date,
    amount,
    LAST_VALUE(amount IGNORE NULLS) OVER (ORDER BY date) AS filled_amount
FROM sales_data
```

---

## 性能优化建议

1. **减少 PARTITION BY 分组数量**：分组越多，计算越慢
2. **合理使用窗口范围**：避免全窗口扫描
3. **先过滤再窗口**：WHERE 条件前置，减少数据量
4. **避免嵌套窗口函数**：多次调用会重复计算

```sql
-- 推荐：先过滤
SELECT 
    *,
    ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC) AS rn
FROM employees
WHERE hire_date >= '2024-01-01'  -- 先过滤，减少数据量

-- 不推荐：先窗口再过滤（窗口函数在全部数据上执行）
SELECT *
FROM (
    SELECT *, ROW_NUMBER() OVER (...) AS rn
    FROM employees
) t
WHERE hire_date >= '2024-01-01'
```