What if fields are not unique?

If the fields used for joining are not unique, the join command will produce a Cartesian product for those non-unique keys. This means that for each matching key in the first file, it will pair with every matching key in the second file.

Example

Consider the following two files:

file1.txt:

1 apple
1 banana
2 cherry

file2.txt:

1 red
1 green
2 yellow

If you run the command:

join file1.txt file2.txt

Output

The output will be:

1 apple red
1 apple green
1 banana red
1 banana green
2 cherry yellow

Explanation

• For the key 1, there are two entries in file1.txt (apple and banana) and two entries in file2.txt (red and green). The join command produces all combinations of these entries.
• For the key 2, there is one entry in each file, so it produces a single output line.

Conclusion

When using non-unique fields for joining, be aware that the output can grow significantly due to the Cartesian product effect, which may lead to a larger result set than expected.