我们可以尝试以下方法，使用TIMESTAMPDIFF()沿着CASE表达式来处理月份的渲染：

SELECT
    USERID,
    CONCAT(TIMESTAMPDIFF(YEAR, date_hired, date_separated), ' Years',
           CASE WHEN TIMESTAMPDIFF(MONTH, date_hired, date_separated) % 12 > 0
                THEN CONCAT(' and ',
                            TIMESTAMPDIFF(MONTH, date_hired, date_separated) % 12,
                            ' months')
                ELSE '' END) AS TENURE
FROM yourTable
ORDER BY USERID;

Demo