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

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 title,avg(salary) avg from titles t left join salaries s on t.emp_no = s.emp_no where t.to_date = '9999-01-01' and  s.to_date = '9999-01-01' group by title