Sort Datetime and Varchar Challenge

Question

Hi! I want to sort datetime (converted to varchar) and varchar together , here is test data generation sql.
SELECT CONVERT(VARCHAR(10),GETDATE()-3,111) AS TEST_DATE INTO #TMP_TEST
INSERT INTO #TMP_TEST SELECT CONVERT(VARCHAR(10),GETDATE()-5,111)
INSERT INTO #TMP_TEST SELECT CONVERT(VARCHAR(10),GETDATE()-7,111)
My expected result should be date in descending order and 'Not Specified' on bottom.
I've tried following sql but 'Not Specified' always on top.
SELECT * FROM #TMP_TEST
UNION ALL
SELECT 'Not Specified'
ORDER BY TEST_DATE DESC
A very bad workaround is to add a space in the beginning of 'Not Specified', like this ' Not Specified' and the result works.
Just want to consult if there is a better way to meet my requirement. 
BTW, TEST_DATE list would be used in Reporting Service query criteria, let user pick a date to query. 'Not Specified' means query ALL TEST_DATE.
Thank you in advance.

Answer 1

Check this conditional order by statement:
SELECT  CONVERT(VARCHAR(10),GETDATE()-3,111) AS TEST_DATE INTO #TMP_TEST
INSERT INTO #TMP_TEST SELECT  CONVERT(VARCHAR(10),GETDATE()-5,111)
INSERT INTO #TMP_TEST SELECT  CONVERT(VARCHAR(10),GETDATE()-7,111)
INSERT INTO #TMP_TEST SELECT  CONVERT(VARCHAR(10),GETDATE()-6,111)
INSERT INTO #TMP_TEST SELECT  CONVERT(VARCHAR(10),GETDATE()-1,111)
INSERT INTO #TMP_TEST SELECT  'Unknown'
SELECT * FROM #TMP_TEST ORDER BY CASE WHEN ISDATE(TEST_DATE) = 1 THEN TEST_DATE END DESC