开发报表时将已有User做成下拉列表，第一项为label为ALL，value为null

SELECT 'All' AS LABLE_NAME, NULL AS USER_NAME
UNION ALL
SELECT USER_NAME AS LABLE_NAME, USER_NAME from dbo.USERS

将ALL作为下拉列表的默认值

这样判断User的时候可用USERNAME=ISNULL(@USERNAME,USERNAME)