Question

I need to join two tables called MSISDN and RANGES.
Example:

MSISDN table:

MSISDN          NETWORK
110011500   D000
110211501   D000

RANGES table:

PREFIX  NETWORK DESCRIPTION
11  D000    NEOTEL
1102    D000    TELKOM

I’m joining as:

select * 
from MSISDN a 
  left join RANGES b 
    on  a.msisdn like b. prefix || '%' 
    and a.network = b.network;

This query will give me duplicates in case as for MSISDN ‘110211501’, because it will match with both prefixes.
I need to avoid these duplicates. I need to only have the match with the longest prefix (in this case ‘1102’).
Is there any way how to join avoiding the duplicates or doing de duplication afterwards?

Answer 1

Try the following query:

SELECT * FROM ( 
    SELECT 
        n.*, 
        RANK() OVER (PARTITION BY n.MSISDN ORDER BY length(n.prefix) DESC) as rnk
    FROM (
        SELECT * 
        FROM MSISDN a 
        LEFT JOIN RANGES b 
             ON a.msisdn like b. prefix || '%' and a.network = b.network 
        ) n 
    )t
WHERE t.rnk = 1

it should remove the duplicates and give you the records with longer prefix.