数据库SQL实战（牛客网）：统计出当前各个title类型对应的员工当前薪水对应的平均工资

统计出当前各个title类型对应的员工当前（to_date=‘9999-01-01’）薪水对应的平均工资。结果给出title以及平均工资avg。
CREATE TABLE salaries (
emp_no int(11) NOT NULL,
salary int(11) NOT NULL,
from_date date NOT NULL,
to_date date NOT NULL,
PRIMARY KEY (emp_no,from_date));
CREATE TABLE IF NOT EXISTS “titles” (
emp_no int(11) NOT NULL,
title varchar(50) NOT NULL,
from_date date NOT NULL,
to_date date DEFAULT NULL);

select a.title , avg(b.salary) as avg
from titles as a join salaries as b
on a.emp_no = b.emp_no
where a.to_date='9999-01-01' and b.to_date='9999-01-01'
group by title;