SQL, joining the same table

Question

I have a table with the columns code and document . The column code may have alphanumeric values (only letters) and/or numeric values (only digits). In the model, every numeric code has an alphanumeric equivalent code . The records below represent an example of this situation (in the form ( document , code );( document , code ):

(12345678900,ABC);(12345678900,999)

But, a alphanumeric code may not always have a equivalent numeric code , so the example below represents a situation where we have 3 different records

(12345678900,ABC);(12345678900,999);(00987654321,XYZ);(11111111111,DEF)

With this in mind, what I want to do is the following: what I'll use to search records is always alphanumeric code s, and when I have an equivalent numeric, I want as result the numeric one, but when the alpha doesn't have the numeric equivalent, I want the alphanumeric code .

For instance, if I execute the following selects, I would get the results below:

1. SELECT code FROM table WHERE code = 'ABC' -> Result: 999
2. SELECT code FROM table WHERE code = 'DEF' -> Result: DEF
3. SELECT code FROM table WHERE code = 'XXX' -> Result: (blank)

I appreciate if anybody could help me please.

Regards,

AMR

Answer 1

Try this

SELECT
    f1.doc
    , COALESCE(f2.code, f1.code) code
FROM foo f1
LEFT JOIN(
    SELECT doc, code
    FROM foo
    WHERE code <> @code
) f2 ON f2.doc = f1.doc
WHERE f1.code = @code

demo