SELECT columns FROM tables
WHERE condition
ORDER BY
case when Table1.Col1 IS NULL then 0 else 1 end ASC
,case when Table1.Col1 IS NULL then Table2.Col2 else Table1.Col1 end DESC
SELECT columns FROM tables
WHERE condition
ORDER BY
case when Table1.Col1 IS NULL then 0 else 1 end ASC
,case when Table1.Col1 IS NULL then Table2.Col2 else Table1.Col1 end DESC