Learn English – drop the second “having” in this sentence

1. The items were entered into the database and the results compared...

2. The items were entered into the database and the results were compared...

There is no doubt whatsoever that both of these sentences are grammatical. They involve an ellipsis of the passive auxiliary were. The ellipsis of auxiliaries in coordinated sentences is quite a common and natural. Indeed we quite often see auxiliaries and lexical verbs elided. However, the problem here is that the sentence can garden path your reader. What this means is that the reader may start interpreting the second clause in a different way from intended, and then, when they get to the end of the sentence, they may have to go back and re-parse what they've read.

In particular the ellipsis of the passive auxiliary in this particular example means that the subject occurs next to what looks like a tensed (past simple) verb form. In actual fact it is a past participle but the reader won't know this till they get to the end of the sentence (and perhaps backtrack and reread). The second clause begins:

• ... the results compared

This may well read as if the results were doing the comparing - as opposed to somebody was comparing the results. Whilst this is not a fatal problem once we get to the end of the sentence, the editor involved may feel that it is a problem best avoided for the sake of ease of reading.