Load Data from a denormalized table to another table - PostgreSQL11

Question

I have a Denormalized Table (say TableA) with the following Columns:

TableA_Id
Cat1
Cat2
Cat3
Cat4
Cat5
Cat6

With entries like:

TableA_ID | Cat1 | Cat2 | Cat3 | Cat4 | Cat5 | Cat6
   1      |  32  |  29  | NULL | NULL | NULL | NULL
   2      |  30  |  56  | 89   | NULL | NULL | NULL
   3      |  32  |  NULL| NULL | NULL | NULL | NULL
   4      |  55  |  65  | 32   | 69   |  3   |  9

I want to convert it into another Table (say TableB) with contains unique associations b/w TableA_ID and the Cat_IDs.

TableB structure

Assoc_Id serial,
TableA_ID int,
Cat_ID    int;

And TableB will have association entries (from TableA) Like:

Assoc_Id | TableA_ID | Cat_ID
  1      |    1      |  32
  2      |    1      |  29
  3      |    2      |  30
  4      |    2      |  56
  5      |    2      |  89
  6      |    3      |  32
  7      |    4      |  55
  8      |    4      |  65
  9      |    4      |  32
  10     |    4      |  69
  11     |    4      |  3
  12     |    4      |  9

Can someone please help?

Thanks in advance.

Answer 1

Elegance is good, but sometimes simple brute force is sufficient. Esp. on what should be a 1 time, or very few executions.

insert into TableB (TableA_id, Cat_id)
     select TableA_id, cat1 from tableA where cat1 is not null  union all
     select TableA_id, cat2 from tableA where cat2 is not null  union all
     select TableA_id, cat3 from tableA where cat3 is not null  union all
     select TableA_id, cat4 from tableA where cat4 is not null  union all
     select TableA_id, cat5 from tableA where cat5 is not null  union all
     select TableA_id, cat6 from tableA where cat6 is not null ;