i have a query similar to this, 

```
SELECT Col_a, Col_b, 0 AS Col_c FROM table1
WHERE confitions 

UNION 

SELECT Col_a, NULL AS Col_b, Col_c FROM table2
WHERE confitions 

ORDER BY CASE WHEN Col_a='Other' THEN 1 ELSE 0 END , 
Col_a

```

but when i run this query it gives me an error: `ORDER BY items must appear in the select list if the statement contains a UNION, INTERSECT or EXCEPT operator. Severity 16`

and if i remove the `case` statement it works fine, can any one help me to solve this issue, thanks in advance.