Print Distinct Values from Field AWK

Question

I'm looking for a way to print the distinct values in a field while in the command-prompt environment using AWK.

ID     Title     Promotion_ID     Flag
12     Purse       7               Y
24     Wallet      7               Y
709    iPhone      1117            Y
74     Satchel     7               Y
283    Xbox        84              N

Ideally I'd like to return the promotion_ids: 7, 1117, 84. I've researched the question on Google and have found some examples such as:

`cut -f 3 | uniq *filename.ext*`               (returned error)
`awk cut -f 3| uniq *filename.ext*`            (returned error)
`awk cut -d, -f3 *filename.ext* |sort| uniq`   (returned error)

Answer 1

awk 'NR>1{a[$3]++} END{for(b in a) print b}' file

Output:

7
84
1117

Answer 2

with the pipe line

$ sed 1d file    |    # remove header
  tr -s ' ' '\t' |    # normalize space delimiters to tabs
  cut -f3        |    # isolate the field
  sort -nu            # sort numerically and report unique entries

7
84
1117