Count how many instances the value in a column appears in that column

Question

I need to create a virtual column in APPSHEET that will return a count of how many times the value in [ptnum] appears in the data table column [ptnum]. I want it to return the number as value in the virtual column. I need an APP Formula example

ptnum	virtual_column
123	3
123	3
222	1
123	3

Answer 1

This formula counts all entries where ptnum is equal to this rows ptnum:

COUNT(
 SELECT(
  TABLE[ptnum],
  [ptnum] = [_THISROW].[ptnum]
 )
)