mysql的group by函数怎么使用

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

1. 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。
2. 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。