Aren Cambre <aren(at)arencambre(dot)com> wrote:
> SELECT COUNT(*)
> FROM consistent.master
> WHERE citation_id IS NOT NULL
> UNION
> SELECT COUNT(*)
> FROM consistent.master
> UNION
> SELECT COUNT(*)
> FROM consistent.master
> WHERE citation_id IS NULL
>
> I got this result:
>
> 2085344
> 2085343
> 0
>
> Not clear how adding a WHERE clause, whose only practical effect
> is to reduce the number of rows returned, could cause *more* rows
> to be returned. That seems buggy to me.
Never assume that the rows will be returned in any particular order
from a query unless you specify ORDER BY. Assuming, as you seem to
be doing, that rows from the left side of a UNION will be output
before rows from the right side is not safe. You have no way of
knowing which row in a result set like that came from which of the
UNIONed SELECTs. In this case your assumption is almost certainly
wrong.
-Kevin