您可以尝试以下步骤.

更新的查询

declare @userid int = -1
if (@userid = -1)
    BEGIN

     SELECT * FROM mytable 
     where Completion_Date>= '11/01/2011' 
     and Completion_Date<= '12/11/2012' 
     and userid in 
         (select distinct userID from mytable)
    end
ELSE
    BEGIN

     SELECT * FROM mytable 
     where Completion_Date>= '11/01/2011' 
     and Completion_Date<= '12/11/2012' 
     and userid = @userid 

end;

结果:

USERID  NAME    COMPLETION_DATE
123     john    2011-11-01
125     tim     2011-11-02
127     ron     2011-11-08

• 参考 SQLFIDDLE

查看特定用户:

• 参考SQLFIDDLE

根据 OP 的最新评论更新

Updated after latest comment from OP

• SQLFIDDLE

查询:

DECLARE @uid int = -1
SELECT 
*
FROM mytable 
WHERE 
( CASE 
     WHEN @uid <> -1 THEN @uid
     ELSE userid
  END
) = userid

and Completion_Date>= '11/01/2011' 
     and Completion_Date<= '12/11/2012' 
;

结果:当@uid = -1

Results: when @uid = -1

USERID  NAME    COMPLETION_DATE
123     john    2011-11-01
125     tim     2011-11-02
127     ron     2011-11-08

如果您尝试过，请发表评论:)

Please comment if you have tried this out :)