'NULL' value can not replaced in sub query

[prabirchoudhury]

CRITERIA;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| CritCode    | int(4)       | NO   | PRI | 0       |       |
| Description | varchar(150) | YES  |     | NULL    |       |
| CritGroup   | varchar(10)  | YES  |     | NULL    |       |
| Detail      | varchar(30)  | YES  |     | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
 
 
SCHCRIT;
+------------+--------+------+-----+---------+-------+
| Field      | Type   | Null | Key | Default | Extra |
+------------+--------+------+-----+---------+-------+
| SchemeCode | int(6) | NO   | PRI | 0       |       |
| CritCode   | int(4) | NO   | PRI | 0       |       |
+------------+--------+------+-----+---------+-------+
 
SCHEMES;
+------------------------+---------------+------+-----+---------+-------+
| Field                  | Type          | Null | Key | Default | Extra |
+------------------------+---------------+------+-----+---------+-------+
| SchemeCode             | int(6)        | NO   | PRI | 0       |       |
| SourceOrg              | int(6)        | YES  |     | NULL    |       |
| Title                  | varchar(100)  | NO   | MUL |         |       |
| Objectives             | mediumtext    | YES  |     | NULL    |       |
| UpdateDate             | date          | YES  |     | NULL    |       |
| CheckDates             | char(1)       | YES  |     | NULL    |       |
| DecisionMakers         | mediumtext    | YES  |     | NULL    |       |
| DecisionTime           | mediumtext    | YES  |     | NULL    |       |
| GroupIndividuals       | char(1)       | YES  |     | NULL    |       |
| EligibleDistricts      | mediumtext    | YES  |     | NULL    |       |
| Eligibility            | mediumtext    | YES  |     | NULL    |       |
| organisationID         | int(11)       | YES  |     | NULL    |       |
+------------------------+---------------+------+-----+---------+-------+
 
Main query: 
select DISTINCT d.title, d.schemecode, d.objectives, 
       d.eligibility, d.eligibledistricts
 
      ,( select DISTINCT COUNT(cr.CritCode) AS Rank 
         from CRITERIA cr 
         INNER JOIN SCHCRIT sc on sc.CritCode=cr.CritCode 
        where sc.SchemeCode= a.schemecode 
       AND cr.CritCode in(71,5043, 5074, 5024, 5025, 4003, 0)) AS match_rank 
      ,(select IF(cr.Description) IS NULL,'XX',cr.Description)
        from CRITERIA cr 
        INNER JOIN SCHCRIT sc on sc.CritCode=cr.CritCode where sc.SchemeCode= a.schemecode 
        AND cr.CritCode = 400 ) AS cost1  
 
 FROM SCHCRIT a inner join SCHCRIT b on(a.schemecode=b.schemecode) 
       inner join SCHCRIT c on(a.schemecode=c.schemecode) 
       inner join SCHEMES d on(a.schemecode=d.schemecode) e) 
       inner join personOrganisation po on (d.sourceorg=po.personid)) 
 WHERE (a.critcode =1 or a.critcode =71)
       and c.critcode in (4000, 4003, 0, 0, 0) 
       and b.critcode in (5000, 5043, 5074, 5024, 5025) 
       order by match_rank DESC, cost1 ASC
 
subquery: this the part of main query mention above and here is having problem with
select IF(cr.Description) IS NULL,'XX',cr.Description)
        from CRITERIA cr 
        INNER JOIN SCHCRIT sc on sc.CritCode=cr.CritCode where sc.SchemeCode= a.schemecode 
        AND cr.CritCode = 400 ) AS cost1

Activity:

I have three tables to pull data from when user is making a search with different 'CRITERIA' those are attached with
different 'SCHEMES'. now i am dysplayning searched results with different sort order.
1. Order by maximum CRITERIA matched SCHEMES ( that i am getting from the subquery result field "AS match_rank" )
2. then 2nd sort is on "cost1" that sorting among given "match_rank". cost1 is coming with some null values those are coming in the top
when i do the sorting in present.
3. both sorting are associated with my main query.


Problem: my main query working fine and fetching all expecting data, but in the second sort on cost1 (that i mentioned above)
"null" value fields are coming onthe top but i wanted them in the bottom. then i tried with different functions to replace NULL value
to bring those end of the sesond sorting but null cant being replaced.

i tryed those differet way with subquery

1. if((field IS NULL), 'XX', field) 
2. ISNULL(field),'XX',field 
3. CASE WHEN field IS NULL then 'XX' else field end 
4. order by case when isNULL(cr.Description) then 1 else 0 end, cr.Description asc 
4. IF(field IS NULL,'XX',field) 

but none of those replacing NULL value

please help me on that.

Note:
1. main query and sub query working fine and giving expected result
2. ISNULL(), IF(), CASE statement working if i use them separately and able to replace NULL but
not as a subquery

thanks in advance