To do this properly, you may want to distinguish between the case where there are actual NULL
results in the data you're summing, and the case where there are no values at all to sum.
Suppose we have the following:
mysql> select * from t;
+----------+------+
| descr | num |
+----------+------+
| hiya | 5 |
| hi there | 10 |
| yo | NULL |
+----------+------+
We would like empty sums to be zero, but sums involving NULL
to be NULL
. One (rather torturous) way to do that is:
mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
-> SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
-> FROM t WHERE num < 'ciao')
-> AS u;
+------+
| sum |
+------+
| 0 |
+------+
1 row in set, 1 warning (0.00 sec)
mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
-> SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
-> FROM t)
-> AS u;
+------+
| sum |
+------+
| NULL |
+------+
1 row in set (0.00 sec)
mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
-> SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
-> FROM t WHERE descr < 'namaste')
-> AS u;
+------+
| sum |
+------+
| 15 |
+------+
1 row in set (0.00 sec)
mysql> SELECT IF(has_null, NULL, total) AS sum FROM (
-> SELECT COALESCE(MAX(num IS NULL), 0) AS has_null, COALESCE(SUM(num), 0) AS total
-> FROM t WHERE descr > 'namaste')
-> AS u;
+------+
| sum |
+------+
| NULL |
+------+
1 row in set (0.00 sec)
Perhaps there's a better way I haven't thought of.
Unfortunately, the SQL standard defines SUM
to be null when no elements are summed, and MySQL has no choice but to follow that standard.