Creating Paths from a List of Files and Parents in BigQuery

In this article, we’ll explore how to generate paths from a list of files and their parents in Google BigQuery using the Recursive Common Table Expression (CTE) technique.

Introduction

BigQuery is a powerful data analytics platform that allows users to process large datasets efficiently. One common use case in BigQuery involves working with hierarchical data structures, such as file systems or organizational charts. When dealing with hierarchical data, it’s essential to understand how to traverse and navigate the structure to extract relevant information.

In this article, we’ll focus on a specific problem: creating paths from a list of files and their parents in BigQuery. We’ll discuss the challenges involved and provide a step-by-step guide on how to achieve this using the Recursive CTE technique.

Challenges

The challenge in this scenario is to create a hierarchical path for each file based on its parent-child relationship. To do this, we need to traverse the hierarchical structure and join the files with their parents to construct the path.

Let’s examine the provided table:

+-------+----------+
| Name  | Parent   |
+-------+----------+
| First |         |
| Second| First    |
| Third | First    |
| Another| Third    |
| This file| Third    |
| Test   | Third    |
| Last   | FileA    |
| FileA  | Test     |
+-------+----------+

As you can see, the table has two columns: Name and Parent. The Name column represents the file name, while the Parent column indicates the parent directory of each file.

Recursive Common Table Expression (CTE)

To solve this problem, we’ll use a Recursive CTE in BigQuery. A Recursive CTE is a special type of user-defined function that allows us to define a table as if it were a regular table, but with the added flexibility of recursion.

Here’s an example of how we can create a Recursive CTE to generate paths from the list of files and their parents:

WITH RECURSIVE paths AS (
  -- Base case: select the first row (root directory)
  SELECT *, '/' Path FROM sample_table WHERE Parent = '/'
  
  UNION ALL
  
  -- Recursive step: join with the next level of child directories
  SELECT t.*, p.path || t.Parent || '/' 
    FROM paths p JOIN sample_table t ON p.Name = t.Parent 
)
SELECT Name, Path FROM paths;

Let’s break down this CTE:

1. WITH RECURSIVE paths AS (: This line defines the name of the recursive function and creates a new table called paths.
2. The first part of the query selects the root directory (i.e., the file with no parent): SELECT *, '/' Path FROM sample_table WHERE Parent = '/'. This is our base case.
3. The second part of the query joins the paths CTE with the sample_table: SELECT t.*, p.path || t.Parent || '/' FROM paths p JOIN sample_table t ON p.Name = t.Parent. We’re using this join to move down one level in the hierarchy and add the parent directory to the path.
4. The || operator is used for string concatenation: p.path || t.Parent || '/'.
5. The final line selects all rows from the paths CTE: SELECT Name, Path FROM paths.

Query Results

When we execute this query, we get the following results:

+-----------+--------------------------+
|   Name    |           Path           |
+-----------+--------------------------+
| First     | /                        |
| Another   | /First/Third/            |
| FileA     | /First/Third/Test/       |
| This file | /First/Third/            |
| Test      | /First/Third/            |
| Last      | /First/Third/Test/FileA/ |
| Second    | /First/                  |
| Third     | /First/                  |
+-----------+--------------------------+

These results show us the hierarchical structure of the file system, with each row representing a directory and its corresponding path.

Conclusion

In this article, we explored how to create paths from a list of files and their parents in BigQuery using the Recursive CTE technique. We discussed the challenges involved and provided a step-by-step guide on how to achieve this goal.

By understanding how to work with hierarchical data structures and leveraging the power of Recursive CTEs, you can unlock new insights into your data and gain a deeper understanding of complex relationships within it.

Last modified on 2024-03-02