My Boss Beats Me

I thought I'm pretty good with sql. Was presented with a little
challenge the other day, a colleague asked me to create a parent to
child relationship for a table without changing db schema. The table
looks like this
pkID parentID altPKfield
1 null abc
2 1 def

It's obvious parent to child relationship via pkID and parentID columns
would do a perfect job, however, he specifically asked not to use pkID
but altPKfield instead.

My first thought was to create a sort of 'ghost column' for 'alt parent
field', then, it goes like using a UDF to get pkID value, use temp
table or table variable, it worked but took quite a bit of time.

The next day, the colleague told me our boss has another solution, that
is, he joins this same table twice to geneate a fourth column of
another pkID, and it works perfect and it's very elegant compared to my
approach.

Question here is, would you know/use his approach without learning this
techque? Second question is, would there be any downside on using
this alternative approach of creating a parent to child relationship
for the same table not using PK column?


Thanks.