mysql中的group by子句用于将查询结果按一个或多个列进行分组，通常与聚合函数（如count, sum, avg, max, min等）配合使用，实现数据的分组统计。以下是详细使用方法：
1. 基础语法
select column1, aggregate_function(column2) from table_name where condition group by column1 order by column1;
2. 核心功能
(1) 单列分组
统计每个部门的员工数量：
select department, count(*) as employee_count from employees group by department;
(2) 多列分组
统计每个部门+职位的平均工资：
select department, job_title, avg(salary) as avg_salary from employees group by department, job_title;
(3) 结合聚合函数
count()：计数sum()：求和avg()：平均值max()/min()：极值group_concat()：合并分组内的字符串
3. 过滤分组：having
where过滤行，having过滤分组：
select department, avg(salary) as avg_salary from employees group by department having avg_salary > 5000; -- 筛选平均工资>5000的部门
4. 常见错误
(1) select非聚合列未分组
❌ 错误写法：
select department, employee_name, count(*) from employees group by department;
✅ 正确写法：
select department, group_concat(employee_name), count(*) from employees group by department;
(2) having与where混淆
where在分组前过滤（如过滤原始数据）having在分组后过滤（如过滤统计结果）
5. 高级用法
(1) with rollup（生成小计/总计）
select department, sum(salary) from employees group by department with rollup;
结果会多出一行null，表示所有部门的总计。
(2) 分组后排序
select department, count(*) from employees group by department order by count(*) desc; -- 按员工数量降序排列
6. 注意事项
- mysql 5.7+默认启用
only_full_group_by模式，要求select中的非聚合列必须出现在group by中。 - 可通过
any_value()函数绕过严格模式：select any_value(employee_name), department, count(*) from employees group by department;
示例场景
统计2023年每月销售额及订单数：
select
month(order_date) as month,
sum(amount) as total_sales,
count(order_id) as order_count
from orders
where year(order_date) = 2023
group by month(order_date)
order by month;通过灵活组合group by和聚合函数，可以高效实现复杂的数据统计分析需求。
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