Add one-skill
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Xin Wang
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# 聚合模式速查
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## 基本聚合
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### 聚合函数列表
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| 函数 | 说明 | 示例 |
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|------|------|------|
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| COUNT(*) | 计数（含NULL行） | 总行数 |
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| COUNT(col) | 计数（不含NULL） | 有效数据数 |
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| COUNT(DISTINCT col) | 去重计数 | 用户数 |
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| SUM(col) | 求和 | 总销售额 |
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| AVG(col) | 平均值 | 平均薪资 |
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| MIN(col) | 最小值 | 最小年龄 |
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| MAX(col) | 最大值 | 最高分 |
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### 基本用法
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```sql
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-- 单列聚合
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SELECT
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COUNT(*) AS total_rows,
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COUNT(DISTINCT user_id) AS unique_users,
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SUM(amount) AS total_amount,
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AVG(amount) AS avg_amount,
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MIN(amount) AS min_amount,
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MAX(amount) AS max_amount
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FROM orders
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-- 分组聚合
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SELECT
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department,
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COUNT(*) AS employee_count,
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AVG(salary) AS avg_salary,
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MAX(salary) AS max_salary
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FROM employees
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GROUP BY department
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```
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---
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## GROUP BY
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```sql
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-- 单字段分组
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SELECT
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category,
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COUNT(*) AS count
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FROM products
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GROUP BY category
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-- 多字段分组
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SELECT
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category,
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brand,
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COUNT(*) AS count,
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SUM(price) AS total_price
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FROM products
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GROUP BY category, brand
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-- 分组 + 排序
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SELECT
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department,
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COUNT(*) AS count
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FROM employees
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GROUP BY department
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ORDER BY count DESC
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-- 分组 + LIMIT（取Top N组）
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SELECT
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category,
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COUNT(*) AS count
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FROM products
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GROUP BY category
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ORDER BY count DESC
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LIMIT 10
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```
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---
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## HAVING（分组过滤）
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```sql
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-- HAVING vs WHERE
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-- WHERE：过滤原始行（GROUP BY 前）
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-- HAVING：过滤分组结果（GROUP BY 后）
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-- 示例：筛选订单数大于10的用户
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SELECT
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user_id,
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COUNT(*) AS order_count,
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SUM(amount) AS total_amount
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FROM orders
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GROUP BY user_id
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HAVING COUNT(*) > 10
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-- 多条件 HAVING
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SELECT
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department,
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AVG(salary) AS avg_salary
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FROM employees
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GROUP BY department
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HAVING AVG(salary) > 5000
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AND COUNT(*) >= 5
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-- HAVING + ORDER BY
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SELECT
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user_id,
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COUNT(*) AS order_count
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FROM orders
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GROUP BY user_id
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HAVING COUNT(*) >= 5
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ORDER BY order_count DESC
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```
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---
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## 去重计数
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```sql
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-- COUNT(DISTINCT)
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SELECT
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COUNT(DISTINCT user_id) AS unique_users
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FROM orders
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-- 分组去重计数
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SELECT
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date,
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COUNT(DISTINCT user_id) AS unique_users,
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COUNT(*) AS total_orders
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FROM orders
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GROUP BY date
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-- 多字段去重计数
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SELECT
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COUNT(DISTINCT user_id, product_id) AS unique_user_product_pairs
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FROM order_items
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-- 大数据量近似去重（性能优化）
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SELECT
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approx_count_distinct(user_id) AS approx_unique_users
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FROM orders
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```
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---
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## 集合聚合（数组结果）
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```sql
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-- collect_list：收集为数组（不去重）
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SELECT
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user_id,
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collect_list(product_id) AS products
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FROM orders
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GROUP BY user_id
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-- collect_set：收集为数组（去重）
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SELECT
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user_id,
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collect_set(product_id) AS unique_products
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FROM orders
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GROUP BY user_id
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-- 取数组大小
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SELECT
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user_id,
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size(collect_list(product_id)) AS product_count,
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size(collect_set(product_id)) AS unique_product_count
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FROM orders
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GROUP BY user_id
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```
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---
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## 多级聚合（ROLLUP / CUBE / GROUPING SETS）
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### ROLLUP（层级汇总）
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```sql
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-- 从右到左递减分组级别
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SELECT
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COALESCE(year, '总计') AS year,
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COALESCE(month, '全年') AS month,
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COALESCE(region, '全国') AS region,
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SUM(sales) AS total_sales
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FROM sales_data
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GROUP BY ROLLUP (year, month, region)
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-- 结果包含：
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-- 1. year + month + region 分组
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-- 2. year + month 汇总（region为NULL）
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-- 3. year 汇总（month,region为NULL）
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-- 4. 全表汇总（year,month,region为NULL）
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```
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### CUBE（全维度组合）
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```sql
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-- 所有分组组合
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SELECT
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COALESCE(year, '总计') AS year,
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COALESCE(month, '全月') AS month,
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COALESCE(region, '全国') AS region,
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SUM(sales) AS total_sales
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FROM sales_data
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GROUP BY CUBE (year, month, region)
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-- 结果包含所有组合：
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-- year+month+region, year+month, year+region, month+region
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-- year, month, region, 全表汇总
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```
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### GROUPING SETS（自定义组合）
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```sql
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-- 指定分组组合
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SELECT
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year,
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month,
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region,
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SUM(sales) AS total_sales
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FROM sales_data
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GROUP BY GROUPING SETS (
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(year, month),
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(year, region),
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(region),
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()
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)
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-- 等价于多个 GROUP BY 合并
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```
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---
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## GROUPING 函数（判断汇总级别）
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```sql
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-- GROUPING(col)：判断该列是否为汇总产生的NULL
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-- 0 = 真实值, 1 = 汇总NULL
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SELECT
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year,
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month,
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SUM(sales) AS total_sales,
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GROUPING(year) AS is_year_total,
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GROUPING(month) AS is_month_total
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FROM sales_data
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GROUP BY ROLLUP (year, month)
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-- 用 GROUPING 区分真实NULL和汇总NULL
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SELECT
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CASE WHEN GROUPING(region) = 1 THEN '全国汇总' ELSE region END AS region,
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SUM(sales) AS total_sales
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FROM sales_data
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GROUP BY ROLLUP (region)
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```
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---
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## 条件聚合（CASE WHEN + 聚合）
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```sql
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-- 分条件统计
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SELECT
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date,
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COUNT(*) AS total_orders,
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SUM(CASE WHEN status = 'completed' THEN 1 ELSE 0 END) AS completed_count,
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SUM(CASE WHEN status = 'cancelled' THEN 1 ELSE 0 END) AS cancelled_count,
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SUM(CASE WHEN status = 'pending' THEN 1 ELSE 0 END) AS pending_count
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FROM orders
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GROUP BY date
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-- 分条件求和
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SELECT
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department,
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SUM(salary) AS total_salary,
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SUM(CASE WHEN gender = 'M' THEN salary ELSE 0 END) AS male_salary,
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SUM(CASE WHEN gender = 'F' THEN salary ELSE 0 END) AS female_salary
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FROM employees
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GROUP BY department
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-- 条件平均值
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SELECT
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category,
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AVG(CASE WHEN price > 100 THEN price ELSE NULL END) AS high_price_avg
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FROM products
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GROUP BY category
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```
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---
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## 聚合 + 窗口函数
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```sql
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-- 分组内占比
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SELECT
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department,
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salary,
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ROUND(salary / SUM(salary) OVER (PARTITION BY department) * 100, 2) AS salary_pct
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FROM employees
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-- 分组累计
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SELECT
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date,
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department,
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amount,
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SUM(amount) OVER (PARTITION BY department ORDER BY date) AS cumulative
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FROM sales
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-- 分组排名
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SELECT
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*,
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RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS salary_rank
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FROM employees
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```
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---
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## 多表聚合
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```sql
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-- JOIN 后聚合
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SELECT
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u.department,
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COUNT(o.id) AS order_count,
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SUM(o.amount) AS total_amount
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FROM users u
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LEFT JOIN orders o ON u.id = o.user_id
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GROUP BY u.department
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-- 子查询聚合
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SELECT
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dept_stats.department,
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dept_stats.avg_salary,
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emp_count.employee_count
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FROM (
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SELECT department, AVG(salary) AS avg_salary
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FROM employees
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GROUP BY department
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) dept_stats
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JOIN (
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SELECT department, COUNT(*) AS employee_count
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FROM employees
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GROUP BY department
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) emp_count ON dept_stats.department = emp_count.department
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```
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---
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## 常见问题
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### 问题1：GROUP BY 字段遗漏
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```sql
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-- 错误：SELECT 字段不在 GROUP BY 中
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SELECT
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department,
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name, -- 错误！name 未分组
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AVG(salary)
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FROM employees
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GROUP BY department
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-- 解决1：添加到 GROUP BY
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SELECT
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department,
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name,
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AVG(salary)
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FROM employees
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GROUP BY department, name
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-- 解决2：使用聚合函数处理
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SELECT
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department,
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collect_list(name) AS names, -- 收集所有name
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AVG(salary)
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FROM employees
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GROUP BY department
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```
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### 问题2：NULL 影响聚合
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```sql
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-- COUNT(*) 包含 NULL 行
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-- COUNT(col) 不包含 NULL
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SELECT
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COUNT(*) AS total, -- 包含 NULL 行
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COUNT(amount) AS valid, -- 不包含 amount 为 NULL 的行
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COUNT(DISTINCT amount) AS unique_values
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FROM orders
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-- SUM/AVG/MIN/MAX 自动忽略 NULL
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SELECT AVG(price) FROM products -- NULL 自动排除
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```
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### 问题3：聚合结果精度
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```sql
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-- AVG 可能精度丢失
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SELECT
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AVG(amount) AS avg_amount -- 可能精度不够
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AVG(CAST(amount AS DECIMAL(18,6))) AS precise_avg -- 高精度
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FROM orders
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-- ROUND 控制精度
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SELECT
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ROUND(AVG(amount), 2) AS avg_amount
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FROM orders
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```
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---
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## 聚合性能优化
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```sql
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-- 1. 先过滤再聚合
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SELECT
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department,
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COUNT(*) AS count
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FROM employees
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WHERE hire_date >= '2024-01-01' -- 先过滤
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GROUP BY department
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-- 2. 大数据量用近似聚合
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SELECT
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approx_count_distinct(user_id) AS users -- 比 COUNT(DISTINCT) 快
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FROM orders
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-- 3. 减少分组字段数量
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SELECT
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category, -- 减少分组字段
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COUNT(*) AS count
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FROM products
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GROUP BY category -- 比 GROUP BY category, brand 快
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-- 4. 避免复杂计算在 GROUP BY 前
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SELECT
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department,
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AVG(salary * 1.1) AS adjusted_avg -- 先计算再聚合
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FROM employees
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GROUP BY department
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```
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---
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## 聚合模式选择指南
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| 需求 | 推荐方式 |
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|------|----------|
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| 简单统计 | GROUP BY + 聚合函数 |
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| 分条件统计 | CASE WHEN + SUM/COUNT |
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| 去重计数 | COUNT(DISTINCT) |
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| 大数据去重 | approx_count_distinct |
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| 收集数组 | collect_list / collect_set |
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| 层级汇总 | ROLLUP |
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| 全维度汇总 | CUBE |
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| 自定义组合 | GROUPING SETS |
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| 分组内计算 | 窗口函数 |
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| 多条件过滤 | HAVING |
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