题解 | 统计各个部门的工资记录数

#不能用内连接要用左连接，因为测试时有另一个例子第一个表重有null值

select a.dept_no,a.dept_name,count(c.salary) sum
from departments a left join dept_emp b on a.dept_no = b.dept_no
left join salaries  c on b.emp_no = c.emp_no
group by a.dept_no


# select d.dept_no,d.dept_name, count(salary) as sum  //简洁明了

# from departments d, dept_emp de, salaries s

# where d.dept_no = de.dept_no

# and de.emp_no = s.emp_no

# group by d.dept_no;