I do NOT think that "union (distinct)" always entails an "order by", neither theoretically (as several others have pointed out), neither practically.

While I've done no testing, there may be cases where SQLServer resorts to hash comparisons insead of ordering to eliminate duplicates. You're probably 99% safe assuming it will "order by" a single int field union. In more complex situations, I wouldn't bet (yet).

Is there any written guarantee from MS that any version of MSSQL always returns ordered results out of "union (disjoint)" queries ? Or are you people just wildguessing ?