>I have an SQL construct.
>
>There are 2 unions in it.
>
>From the interrogation I don't get all the data.
>
>Here's what I did. I cut the part beginning at the first union and then ran the interrogation. All the data was there.
>
>When I add the part with union then data is missing.
>
>I don't get how that could be the case. I mean I get some data and after that I add 2 unions. How can a union give me less data than I add before it?

Did you use UNION ALL or just UNION?
If you use just UNION that construction filter duplicated records.