rounding behaviour

[David Garamond]

hi,

mysql> select 1/29;
+------+
| 1/29 |
+------+
| 0.03 |
+------+
1 row in set (0.00 sec)

mysql> select 100.0*1/29;
+------------+
| 100.0*1/29 |
+------------+
| 3.448 |
+------------+
1 row in set (0.00 sec)

mysql> select 1/29*100.0;
+------------+
| 1/29*100.0 |
+------------+
| 3.45 |
+------------+
1 row in set (0.00 sec)

i am slightly puzzled by mysql's behaviour in the first case. this seems to be a float division, but why does mysql rounds it to two digits after decimal? if this is supposed to be an integer division, wouldn't it be better/more predictable for mysql to return 0?

this behaviour could cause subtle problem/errors because people really didn't expect this kind of behaviour. for example, i just found out today, after weeks of operation, that my sql expression:

SELECT ...,if(sum(CLICKS),sum(IMPS)/sum(CLICKS)*100.0,0) as CTR FROM T

generates CTR that are rounded to two digits after decimal (yes, IMPS and CLICKS are integer fields). however, after i change the expression to this:

SELECT ...,if(sum(CLICKS),100.0*sum(IMPS)/sum(CLICKS),0) as CTR FROM T

all is well.

--
dave


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