How to sort and join at the same time?

Question

I have 2 files, one having 2 columns, another having 1 column.
The second file is sorted using sort -u.
Now the task is I need to join this column with the first column of the first file, which is not sorted.

So what will be the syntax? Will join -j 1 file2.txt sort -s -n -k 1 file1.txt work?

The output I want is actually the 2nd column of file 2 after joining and the unique entries in it.

File 2

---

1
2
3

File 1

---

2  500
1  5000 
1  300
3  3000 
3  300
4  450

Output

---

5000
300
500
3000

Answer 1

No need to use non-standard process substitution (<(...)) here:

sort file1 | join -o1.2 - file2 | uniq

Answer 2

join file2.txt <(sort file1.txt) | awk '{print $2}'

Answer 3

One way using sort + awk. I sort the other file by its first number and in stable mode. In awk I compare what keys from file1 match with the keys of file2 and are not repeated, printing them:

sort -snk1,1 file1 | awk '
    FNR == NR { 
        keys[ $1 ] = 1; 
        next; 
    } 
    !values[ $2 ] && keys[ $1 ] { 
        printf "%s\n", $2; 
        values[ $2 ] = 1; 
    }
' file2 -

Output:

5000
300
500
3000