Multiple-level Mapping (Tree Hierachy) with SQL

Introduction

When dealing with hierarchical data, such as a network log table where each entry has a source IP and a destination IP, it can be challenging to create meaningful queries that extract the desired information. One common requirement is to fetch multiple levels of associations between these IPs, which forms a tree-like structure.

In this article, we will explore how to achieve multiple-level mapping using SQL, specifically focusing on the concept of Common Table Expressions (CTEs) and their application in solving this type of problem.

Understanding Hierarchical Data

To approach this problem, let’s first understand what hierarchical data means. In the context of network logs, each entry represents a request from one IP to another. For instance:

• ip01_1 sends data to ip01_2
• ip01_1 sends data to ip01_3
• ip01_2 sends data to ip01_4
• ip01_4 sends data to ip01_5

These associations form a hierarchical tree, where each node represents an IP and its child nodes represent the subsequent destinations in the request chain.

Self-Joining: A Common Approach

One immediate solution to fetch multiple levels of these associations is by using self-joining. In this approach, we join the table with itself on the dest_ip column to link back to the original source IP. However, as mentioned in the question, self-joining poses a challenge due to its time complexity.

-- Example SQL query for simple self-join (not suitable for large datasets)
SELECT t1.src_ip AS first_src_ip, t2.dest_ip 
FROM table_name t1, table_name t2 
WHERE t1.dest_ip = t2.src_ip;

Using Common Table Expressions (CTEs)

A more efficient approach is to use CTEs. A CTE allows us to create a temporary result set that we can reference within a SELECT, INSERT, UPDATE, or DELETE statement.

In the provided example, three CTEs are used:

• IPs: This CTE defines the original table with multiple levels of associations.
• Hop1 and Hop2: These CTEs perform two levels of self-joining to find the first requesting IP for each destination IP.

Here’s a breakdown of how these CTEs work:

IPs CTE

WITH IPs AS (
  SELECT 'ip01_1' AS src_ip, 'ip01_2' AS dest_ip UNION ALL 
  SELECT 'ip01_1', 'ip01_3' UNION ALL 
  SELECT 'ip01_2', 'ip01_4' UNION ALL 
  SELECT 'ip01_4', 'ip01_5' UNION ALL 
  SELECT 'ip02_1', 'ip02_2' UNION ALL 
  SELECT 'ip02_2', 'ip02_3'
),

The IPs CTE generates the original table with multiple levels of associations. This is done using a union operator (UNION ALL) to combine rows.

Hop1 and Hop2 CTEs

Hop1 AS (
  SELECT 
    COALESCE(
      (SELECT MIN(ip2.src_ip) FROM IPs AS ip2 WHERE ip.src_ip = ip2.dest_ip),
      src_ip
    ) AS src_ip,
    dest_ip
  FROM IPs AS ip
),
Hop2 AS (
  SELECT 
    COALESCE(
      (SELECT MIN(ip2.src_ip) FROM IPs AS ip2 WHERE ip.src_ip = ip2.dest_ip),
      src_ip
    ) AS src_ip,
    dest_ip
  FROM Hop1 AS ip
)

The Hop1 and Hop2 CTEs perform two levels of self-joining to find the first requesting IP for each destination IP.

Here’s how it works:

• The COALESCE function returns the first non-null value. When used with a subquery, it finds the minimum source IP that corresponds to the current destination IP.
• In the Hop1 CTE, we join the IPs table with itself on the dest_ip column and find the minimum source IP for each destination IP.
• The same logic is applied in the Hop2 CTE, but this time starting from the results of the first CTE.

Final Query

SELECT *
FROM Hop2
ORDER BY src_ip;

The final query simply selects all columns (*) from the Hop2 CTE and orders the result by the source IP. This yields a table with multiple levels of associations between source IPs and destination IPs:

Conclusion

Achieving multiple-level mapping with SQL can be challenging due to the hierarchical nature of the data. However, using CTEs provides a powerful solution for this problem. By breaking down the query into manageable steps and leveraging recursive joins, we can efficiently extract meaningful information from complex hierarchical data structures.

This article has demonstrated how to use CTEs in SQL to create multiple levels of associations between source IPs and destination IPs. We hope that this explanation has helped you understand the concept of multiple-level mapping with SQL and provided a practical approach for tackling similar problems in your own projects.

Last modified on 2025-01-03