在同一张表中返回当月薪资和上月薪资

SELECT
    current_month.salary AS 当月薪资,
    previous_month.salary AS 上月薪资
FROM
    salary_table AS current_month
JOIN
    salary_table AS previous_month
ON
    current_month.employee_id = previous_month.employee_id
WHERE
    current_month.month = CURRENT_DATE()
    AND previous_month.month = DATE_SUB(CURRENT_DATE(), INTERVAL 1 MONTH);