【MySQL基础】聚合函数从基础使用到高级分组过滤

# 计算所有员工的工资总和 
select sum(salary) from emp; 
# 计算销售部门的加班费总额 
select sum(overtime_pay) from test where department = 'sales';

# 计算公司平均薪资 
select avg(salary) from epm; 
# 计算技术部平均工龄（保留2位小数） 
select round(avg(years_of_service), 2) from epm where department = 'technology';

# 查询最高和最低工资 
select max(salary), min(salary) from epm; 
# 找出最近和最早上架的商品 
select max(create_time), min(create_time) from products;

# 统计员工总数 
select count(*) from epm; 
# 统计有邮箱的员工数量（不统计NULL） 
select count(email) from epm; 
# 统计不同部门的数量 
select count(distinct department) from epm;

# 统计各数据库占用空间（MB）
select table_schema, 
       round(sum(data_length)/1024/1024, 2) as size_mb
from information_schema.tables 
group by table_schema;

# 按部门统计员工
select department, 
       count(*) as employee_count,
       avg(salary) as avg_salary
from epm
group by department;

# 商品分类统计
select category,
       count(*) as product_count,
       min(price) as min_price,
       max(price) as max_price,
       avg(price) as avg_price
from products
group by category;

# 按部门和职位统计
select department, 
       job_title,
       count(*) as headcount,
       sum(salary) as total_salary
from epm
group by department, job_title;

# 分析慢查询日志，按查询类型统计平均执行时间
select query_type,
       count(*) as query_count,
       avg(execution_time) as avg_time_ms
from slow_query_log
where log_date = curdate()
group by query_type;

# 筛选销售额大于1000的部门
select department, 
       sum(sales) as total_sales
from sales_records
group by department
having sum(sales) > 1000;

# 筛选平均工资高于公司平均的部门
select department,
       avg(salary) as dept_avg_salary
from epm
group by department
having avg(salary) > (select avg(salary) from epm);

# 找出表空间超过1GB的数据库
select table_schema,
       round(sum(data_length)/1024/1024, 2) as size_mb
from information_schema.tables
group by table_schema
having size_mb > 1024
order by size_mb desc;

select department,
       job_title,
       count(*) as headcount
from epm
group by department, job_title with rollup;

• 为GROUP BY列添加索引

alter table epm add index idx_department (department);

• 避免在WHERE中使用聚合函数

# 错误写法
select department, avg(salary) 
from epm 
where avg(salary) > 5000  # 这里会报错
group by department;

# 正确写法
select department, avg(salary) 
from epm 
group by department
having avg(salary) > 5000;

explain 
select department, count(*) 
from epm 
group by department;

# 错误示例
select employee_name, department, count(*)
from epm
group by department;

# 正确写法
select department, count(*)
from epm
group by department;

# 或者包含所有非聚合列
select employee_name, department, count(*) over (partition by department)
from epm;

# COUNT(列名)会忽略NULL，COUNT(*)不会
select count(commission), count(*) 
from epm 
where department = 'sales';

# 使用IFNULL处理
select department, avg(ifnull(commission, 0)) 
from epm
group by department;