Well I finally found the solution. With this syntax:
SQL SELECT 'gabel' omgeving, a.owner owner , a.table_name tabel_naam, b.num_rows num_rows, b.last_analyzed last_analyzed
FROM all_tables@gabel a
where a.owner not in ('SYSTEM', 'SYS')
and a.table_name = b.table_name ;
Qlikview creates the column names in CAPITALS! So my code is now:
,if(count(TABEL_NAAM)<2,-1) as "Only in One Table?"
,if(max(NUM_ROWS)<>min(NUM_ROWS),-1) as "Amount Different?"
GROUP BY TABEL_NAAM
I did CTRL-T to show the table structure and there I saw the capitals.