获取时间
SELECT unix_timestamp()                      //1621909170(当前时间戳：年月日时分秒)
SELECT unix_timestamp(CURDATE())      　　　　//1621872000(当前时间戳：年月日)
SELECT NOW()                                 //2021-05-25 10:20:28(当前时间：年月日时分秒)