mysql中某列前面的'-'是什么意思？

Question

下面代码中的“-”（代码最后一行，a.emp_no之前）是什么意思？我一直在努力理解，但我想要确定的答案。请帮助!!!!!!!!!!!!!!!!!!!!

SELECT *
FROM
(SELECT
    e.emp_no,
    e.first_name,
    e.last_name,
    NULL AS dept_no,
    NULL AS from_date
FROM
    employees.employees e
WHERE
    last_name = 'Denis' 
UNION SELECT
    NULL AS emp_no,
    NULL AS first_name,
    NULL AS last_name,
    dm.dept_no,
    dm.from_date
FROM
   employees.dept_manager dm) as a
ORDER BY -a.emp_no DESC;

Answer 1

如果 emp_no 是一个数字 - 那么它要求按数字的负数排序。你也可以说

order by a.emp_no

mysql中某列前面的'-'是什么意思？

What is '-' in front of a column in mysql mean?

mysql

union