Question

Assuming I have the following table:

AAAAAA
AAAAAB
CCCCCC

How could I craft a query that would let me know that AAAAA and AAAAB are similar (as they share five characters in a row)? Ideally I would like to write this as a query that would check if the two fields shared five characters in a row anywhere in the string but this seems outside the scope of SQL and something I should write into a C# application?

Ideally the query would add another column that displays: Similar to 'AAAAA', 'AAAAB'

Answer 1

I suggest you do not try to violate 1NF by introducing a multi-valued attribute.

Noting that SUBSTRING is highly portable:

WITH T 
     AS
     (
      SELECT * 
        FROM (
              VALUES ('AAAAAA'), 
                     ('AAAAAB'), 
                     ('CCCCCC')
             ) AS T (data_col)
     )
SELECT T1.data_col, 
       T2.data_col AS data_col_similar_to
  FROM T AS T1, T AS T2
 WHERE T1.data_col < T2.data_col
       AND SUBSTRING(T1.data_col, 1, 5) 
              = SUBSTRING(T2.data_col, 1, 5);

Alternativvely:

T1.data_col LIKE SUBSTRING(T2.data_col, 1, 5) + '%';