在 SQL 中，GROUP BY 用于按一个或多个列对查询结果进行分组，通常配合聚合函数（如 COUNT、SUM、AVG、MAX、MIN）使用。
SELECT 列1, 聚合函数(列2)
FROM 表名
[WHERE 条件]
GROUP BY 列1
[ORDER BY ...];
⚠️ 注意：
SELECT 中出现的非聚合列，必须出现在 GROUP BY 中
统计每个部门的员工人数：
SELECT department, COUNT(*) AS emp_count
FROM employees
GROUP BY department;
统计每个部门、每个职位的员工数：
SELECT department, job, COUNT(*)
FROM employees
GROUP BY department, job;
⚠️ WHERE 不能筛选聚合结果，要用 HAVING
查询人数大于 5 的部门：
SELECT department, COUNT(*) AS cnt
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;
SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
ORDER BY avg_salary DESC;
FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY
SELECT name, COUNT(*)
FROM employees
GROUP BY department;
❌ 错误：name 不在 GROUP BY 中，也不是聚合列
避免“隐藏列”分组，写清楚所有分组列。
如果你用的是 MySQL / SQL Server / Oracle / PostgreSQL，或者想结合具体表结构举例，我可以给你写专用 SQL。
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