仍然是一项艰巨的任务...

Still, it is a tough assignment...

 SELECT x.date, MIN(y.date) y_date,DATEDIFF(MIN(y.date),x.date) days
FROM
(
SELECT hiredate date FROM employees
UNION 
SELECT terminationdate FROM employees
) x
JOIN
(
SELECT hiredate date FROM employees
UNION 
SELECT terminationdate FROM employees
UNION
SELECT CURDATE())
y
ON y.date > x.date
GROUP BY x.date
ORDER BY days DESC LIMIT 1;