SELECT role, sum(years_employed)/count(role) as Average_years_employed
FROM employees
GROUP BY role;  与
SELECT role, avg(years_employed)/count(role) as Average_years_employed
FROM employees
GROUP BY role;  为什么第一个查询语句与第二查询语句得出的工龄平均值会不一样，sum(years_employed)/count(role)和avg(years_employed)/count(role)意思是一样的吧，请各位高手帮忙解答下，谢谢！