Why Soundex function does not work for numbers?

We have gone through how amazing Soundex function really is. When used with letters it returns strings that are phonetically equivalent to each other and sounds alike in English. Also, we gained knowledge about the wonderful algorithm that works behind its back. Believe me, if you are a technical developer having an understanding of this function sometimes proves to be extremely useful. But due to some reason Soundex function does not retrieve any output if used with numbers.

Read here to learn how algorithm of Soundex function works

What is the problem of Soundex with numbers then?

Before we figure out reason for why Soundex does not work with numbers let us go through its algorithm first

  1. Retain the first letter of the string
  2. Remove all other occurrences of the following letters: a, e, h, i, o, u, w, y (or change it to zero ‘0’)
  3. Assign digits to the remaining letters (after the first) as follows:
    b, f, p, v = 1
    c, g, j, k, q, s, x, z = 2
    d, t = 3
    l = 4
    m, n = 5
    r = 6
  4. If two or more letters with the same number were adjacent in the original name (before step 1), or adjacent except for any intervening h and w, then omit all but the first.
  5. Replace the first digit with the letter (as in Step 1)
  6. If the string is less than 4 letters pad ‘0’ on the right. If it is more than 4 letters return only the first four positions.

The Reason

If you look closely at Step 2 we can see that a numeric digit is assigned to a letter or set of letters except for those mentioned in Step 1. But for numbers, there is no such assignment of numeric digits.

So if we use numbers as characters in Soundex function there will be nothing assigned to them and query will not retrieve any rows.

For example, below query will give no output:

FROM dual
WHERE Soundex('100') = Soundex('100');

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