Union based on string of identifiers PostGIS

Question

I have a table of documents relating to several parcels of land. I have in this table the identifiers (numerals, sometimes interspaced by "/"- text field, delimited by commas). I also have a land division table that contains these same identifiers and geometries.

What I want is to assign geometry to the entries in this table, based on these identifiers. Where there is just one, no problem exists, because a simple "=" will do the trick. Where the problem is at, is in cases where there is more than one parcel I want to join them in one polygon.

What I have so far only produces an empty row:

SELECT o.*, st_union(a.geometry) as geometry
FROM   "documents" o, parcels a 
WHERE  o.identifier like '%' || a.identifier || '%'

EDIT:

To clarify, my table that I wish to add geometry to (it has none to begin with) has a structure like this:

identifier | f1 | f2 | f3 | f4 ...

201,202,203/1,203/2 | x | x2 | x3 | x4 ...

206 | x | x2 | x3 | x4 ...

While the parcel table is always like:

identifier | f1 | f2 | f3 | geometry ...

206 | x | x2 | x3 | binaryGeometry ...

In the example above, parcels 201,202,203/1,203/2 need to be merged into one polygon. I feel like this kind of task should be easily achievable, yet I can't make it work.