SELECT Tax_Percent FROM Tax_Table TT
WHERE TT.Tax_Type = 'GST'
AND TT.Effective_Date =
  (SELECT MAX(Effective_date)
  FROM Tax_Table T1
  WHERE T1.Tax_type = TT.Tax_Type
  AND T1.Effective_date <= '1999-01-01')

returns 1 row:

0.07