Replace NULL by default value in LEFT JOIN, but not in ROLLUP - mysql

Question

I have a query having LEFT JOIN, group by and ROLLUP, like this:
Select * from
(
Select user_agent,
value,
recoqty,
count(recoqty) as C
from august_2016_search_stats SS
LEFT JOIN august_2016_extra E
on (SS.id = E.stats_id and E.key = 'personalized')
where time >= '2016-08-22 00:00:00' and
time <= '2016-08-22 23:59:59' and
query_type = 'myfeed' and
recoqty = 'topics'
group by recoqty,
user_agent,
value
with ROLLUP
having recoqty is not null
) D
order by C desc;
which gives result like this:
+------------+-------+---------+------+
| user_agent | value | recoqty | C |
+------------+-------+---------+------+
| NULL | NULL | topics | 1330 |
| abscdef | NULL | topics | 1330 |
| abscdef | NULL | topics | 1285 |
| abscdef | 1 | topics | 25 |
| abscdef | 0 | topics | 20 |
+------------+-------+---------+------+
Here, the value (NULL 1285) is due to LEFT JOIN, and the value (NULL 1330) is due to rollup.
However, is there a way to replace NULL value ONLY for LEFT JOIN and not for ROLLUP ?

Answer 1

This is a little tricky, because it appears that the NULL values coming from your data are indistinguishable from the NULL values coming from the rollup. One possible workaround is to first do a non-aggregation query in which you replace the NULL values from the value column with 'NA', or some other placeholder, using COALESCE. Then aggregate this as a subquery using GROUP BY with rollup. Then the NULL values in the value column will with certainty be from the rollup and not your actual data.
SELECT t.user_agent,
t.value,
t.recoqty,
t.C
FROM
(
SELECT user_agent,
COALESCE(value, 'NA') AS value
recoqty,
COUNT(recoqty) AS C
FROM august_2016_search_stats SS
LEFT JOIN august_2016_extra E
ON SS.id = E.stats_id AND
E.key = 'personalized'
WHERE time >= '2016-08-22 00:00:00' AND
time <= '2016-08-22 23:59:59' AND
query_type = 'myfeed' AND
recoqty = 'topics'
) t
GROUP BY t.recoqty,
t.user_agent,
t.value
WITH ROLLUP
HAVING t.recoqty IS NOT NULL