SQL -- GROUP BY 基本语法

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SELECT column1, column2, aggregate_function(column3)
FROM table_name
WHERE condition
GROUP BY column1, column2
ORDER BY column1, column2;

主要用途

GROUP BY用于将结果集按照一个或多个列进行分组，通常与聚合函数（如COUNT、SUM、AVG、MAX、MIN）一起使用。

常用聚合函数

COUNT() - 计算行数
SUM() - 求和
AVG() - 平均值
MAX() - 最大值
MIN() - 最小值
GROUP_CONCAT() - 连接字符串（MySQL）

实际示例

1. 基本分组统计

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-- 按部门统计员工数量
SELECT department, COUNT(*) as employee_count
FROM employees
GROUP BY department;

2. 多列分组

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-- 按部门和职位统计
SELECT department, position, COUNT(*) as count
FROM employees
GROUP BY department, position;

3. 使用聚合函数

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-- 按部门统计平均工资
SELECT department, 
       COUNT(*) as employee_count,
       AVG(salary) as avg_salary,
       MAX(salary) as max_salary,
       MIN(salary) as min_salary
FROM employees
GROUP BY department;

4. 带WHERE条件的分组

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-- 统计2023年各月份的销售总额
SELECT MONTH(order_date) as month,
       SUM(amount) as total_sales
FROM orders
WHERE YEAR(order_date) = 2023
GROUP BY MONTH(order_date)
ORDER BY month;

5. 使用HAVING过滤分组结果

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-- 找出平均工资超过50000的部门
SELECT department, AVG(salary) as avg_salary
FROM employees
GROUP BY department
HAVING AVG(salary) > 50000;

6. 复杂分组示例

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-- 按年份和季度统计销售数据
SELECT 
    YEAR(order_date) as year,
    QUARTER(order_date) as quarter,
    COUNT(*) as order_count,
    SUM(amount) as total_amount,
    AVG(amount) as avg_amount
FROM orders
WHERE order_date >= '2020-01-01'
GROUP BY YEAR(order_date), QUARTER(order_date)
ORDER BY year, quarter;

重要注意事项

1. SELECT子句规则

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-- ✅ 正确：SELECT中的非聚合列必须在GROUP BY中
SELECT department, COUNT(*)
FROM employees
GROUP BY department;

-- ❌ 错误：name不在GROUP BY中
SELECT department, name, COUNT(*)
FROM employees
GROUP BY department;

2. WHERE vs HAVING (？)

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-- WHERE：过滤行（在分组前）
SELECT department, COUNT(*)
FROM employees
WHERE salary > 30000
GROUP BY department;

-- HAVING：过滤分组（在分组后）
SELECT department, COUNT(*)
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;

3. 分组排序

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-- 按分组结果排序
SELECT department, COUNT(*) as emp_count
FROM employees
GROUP BY department
ORDER BY emp_count DESC;

实际应用场景

1. 数据分析

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-- 用户行为分析：按日期统计访问量
SELECT DATE(visit_time) as date,
       COUNT(DISTINCT user_id) as unique_visitors,
       COUNT(*) as total_visits
FROM user_visits
GROUP BY DATE(visit_time)
ORDER BY date;

2. 报表生成

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-- 销售报表：按产品类别统计
SELECT category,
       COUNT(*) as product_count,
       SUM(price * quantity) as total_revenue
FROM sales
GROUP BY category
ORDER BY total_revenue DESC;

3. 数据清理

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-- 查找重复数据
SELECT email, COUNT(*) as count
FROM users
GROUP BY email
HAVING COUNT(*) > 1;

GROUP BY是SQL中非常重要的功能，特别适用于数据分析和报表生成。记住要正确使用WHERE和HAVING，以及确保SELECT子句中的非聚合列都在GROUP BY中。