SELECT e1.first_name
FROM employees e1
WHERE (
SELECT count(*)
FROM employees e2
WHERE e1.first_name >=e2.first_name
)%2=1;
方法2：
SELECT b.first_name
FROM employees AS b,(
SELECT first_name,DENSE_RANK() OVER(ORDER BY first_name) AS t_rank
FROM employees
) AS a
WHERE b.first_name=a.first_name AND a.t_rank%2=1