What are the dictionaries with a single value and multiple keys stands for? What are their purposes?
I've accidentally created one, but can not do anything with it:
q)type (`a`b`c)!(`d)
99h
q)((`a`b`c)!(`d))[`a]
'par
That special form usually denotes the flip of a partitioned table, where the keys represent the column names and the value represents the table name:
q)/load a database with partitioned table part_tab
q)flip part_tab
`ncej`jogn`ciha`hkpb`aeaj`blmj`ooei`jgjm`cflm`bpmc!`part_tab
This dictionary is not intended to be looked up in the usual manner and not in the way that you've attempted.
It would be completely ill-advised but it is possible to restrict columns of a partitioned table by manipulating this dictionary:
q)select from part_tab where date=2020.01.02
date ncej jogn ciha hkpb aeaj blmj ooei jgjm cflm bpmc
------------------------------------------------------------
2020.01.02 0 0 0 0 0 0 0 0 0 0
2020.01.02 1 1 1 1 1 1 1 1 1 1
2020.01.02 2 2 2 2 2 2 2 2 2 2
2020.01.02 3 3 3 3 3 3 3 3 3 3
...
q)part_tab:flip`ncej`jogn`ciha!`part_tab
q)select from part_tab where date=2020.01.02
date ncej jogn ciha
-------------------------
2020.01.02 0 0 0
2020.01.02 1 1 1
2020.01.02 2 2 2
...
Again don't try this on any large/production tables, it's an undocumented quirk.
Splay table have a similar dictionary when flipped:
q)flip splay
`ncej`jogn`ciha`hkpb`aeaj`blmj`ooei`jgjm`cflm`bpmc!`:splay/
The difference being that the table name has a "/" at the end and is hsym'd. This is how .Q.qp determines if a table is partitioned or splayed.