Description:
Using IFNULL() or COALESCE() (and other NULL-detecting functions) in a WITH ROLLUP query
causes the result set to contain bad values for the total/subtotal rows (ie: the NULLs are
replaced with the previous non-NULL value in the column).

How to repeat:
SELECT a FROM ( SELECT 1 a ) t1 GROUP BY a WITH ROLLUP;
SELECT COALESCE(a, 'TEST') FROM ( SELECT 1 a ) t1 GROUP BY a WITH ROLLUP;

SELECT a FROM ( SELECT 1 a UNION SELECT 2 ) t1 GROUP BY a WITH ROLLUP;
SELECT IFNULL(a, 'TEST') FROM ( SELECT 1 a UNION SELECT 2 ) t1 GROUP BY a WITH ROLLUP;

SELECT a, b FROM ( SELECT 1 a, 2 b UNION SELECT 3, 4 ) t1 GROUP BY a, b WITH ROLLUP;
SELECT IFNULL(a, 'TEST'), COALESCE(b, 'TEST') FROM ( SELECT 1 a, 2 b UNION SELECT 3, 4 )
t1 GROUP BY a, b WITH ROLLUP;

Suggested fix:
Should remain NULL in the final output.

the error is in the query:
#group by a,b

the result is groubed by a and b, not by the result of the function, the columns with the
result are not grouped nor aggregate, so they are ignored, by rollup.

better visible in the following query

SELECT a, "fix"
FROM ( SELECT 1 a, 2 b UNION SELECT 3, 4 ) t1
GROUP BY a, b WITH ROLLUP;

nothing groups on "fix".

Sorry, I have been wrong in my last comment, but then it does have nothing do to with the
IFNULL function

select a, a+1 , sum(b) from (select 1 a, 2 b) t group by a with rollup;

a is null, but a+1 = 2

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/internals/25380

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/internals/25382

ChangeSet
  1.2301 05/05/30 03:01:51 igor@rurik.mysql.com +3 -0
  olap.result, olap.test:
    Added test cases for bug #7894.
  sql_select.cc:
    Fixed bug #7894: GROUP BY queries with ROLLUP returned
    wrong results for expressions containing group by columns.
    The fix ensured correct results by replacement of all
    occurrences of group by fields in non-aggregate expressions
    for corresponding ref objects and preventing creation of
    fields in temporary tables for expression containing group
    by fields.

The fix will appear in 4.1.13 and 5.0.7

Documented in 4.1.13 and 5.0.7 changelogs:

<listitem><para>GROUP BY queries with ROLLUP returned
    wrong results for expressions containing group by columns. (Bug
#7894)</para></listitem>