Understanding BigQuery and Array Manipulation

BigQuery is a fully managed data warehousing service by Google Cloud. It allows users to run SQL-like queries on large datasets stored in the cloud. One of the key features of BigQuery is its support for arrays, which are collections of values that can be manipulated like regular columns.

In this article, we’ll focus on how to extract the next value in an array delimited by “->” in BigQuery. This is a common use case when dealing with data that contains nested structures or hierarchies.

Problem Statement

The problem at hand is to take an array of values and return the next value after a specific delimiter “->”. The catch is that this delimiter is used to separate multiple words within the array, making it difficult to extract the desired value.

For example, given the following array:

ROW 1- "Q -> Res -> tes -> Res -> twet"
ROW 2- "rw -> gewg -> tes -> Res -> twet"
ROW 3- "Y -> Res -> Res -> Res -> twet"

We want to extract the next value after “Res” in each row. The output should be:

ROW 1- tes
ROW 2- tewt
ROW 3- tewt

Solution Overview

There are two approaches to solve this problem. We’ll explore both and discuss their strengths and weaknesses.

Approach 1: Using Offset and Trim Functions

The first approach uses the offset and trim functions to extract the next value after “Res”.

SELECT id, 
  (SELECT word FROM UNNEST(arr) word WITH OFFSET
   WHERE offset > (SELECT offset FROM UNNEST(arr) word WITH OFFSET WHERE trim(word) = 'Res' LIMIT 1)
   AND trim(word) != 'Res'
   ORDER BY offset LIMIT 1
  ) AS next_word
FROM your_table, UNNEST([struct(split(path, '->') as arr)])

This approach works by first finding the offset of the “Res” word in the array. It then uses this offset to find the next value that is not equal to “Res”. However, this approach has a weakness: it may return incorrect results if there are multiple instances of “Res” in the same row.

Approach 2: Using Regular Expressions and Trim Functions

The second approach uses regular expressions and the trim function to extract the next value after “Res”.

SELECT id, 
  (SELECT split(pair, ' -> ')[offset(1)]
    FROM UNNEST(arr) pair WITH OFFSET
    WHERE trim(pair) != 'Res -> Res'
    ORDER BY offset LIMIT 1
  ) AS next_word
FROM your_table, UNNEST([struct(regexp_extract_all(path, r' Res -> \w+') as arr)])

This approach works by using a regular expression to extract all words that follow “Res” in the array. It then uses the trim function to remove any leading or trailing spaces from the result.

Choosing the Right Approach

Both approaches have their strengths and weaknesses. The first approach is simpler to understand but may return incorrect results if there are multiple instances of “Res” in the same row. The second approach is more complex but provides a more accurate solution.

In general, it’s recommended to use the second approach when dealing with arrays that contain nested structures or hierarchies. This approach provides more flexibility and accuracy than the first approach.

Example Use Cases

Here are some example use cases for extracting the next value after “Res” in an array:

-- Example 1: Simple array
SELECT id, 
  (SELECT word FROM UNNEST(arr) word WITH OFFSET
   WHERE offset > (SELECT offset FROM UNNEST(arr) word WITH OFFSET WHERE trim(word) = 'Res' LIMIT 1)
   AND trim(word) != 'Res'
   ORDER BY offset LIMIT 1
  ) AS next_word
FROM your_table, UNNEST([struct(split(path, '->') as arr)])

-- Example 2: Array with multiple instances of "Res"
SELECT id, 
  (SELECT split(pair, ' -> ')[offset(1)]
    FROM UNNEST(arr) pair WITH OFFSET
    WHERE trim(pair) != 'Res -> Res'
    ORDER BY offset LIMIT 1
  ) AS next_word
FROM your_table, UNNEST([struct(regexp_extract_all(path, r' Res -> \w+') as arr)])

-- Example 3: Array with nested structures
SELECT id, 
  (SELECT split(pair, ' -> ')[offset(1)]
    FROM UNNEST(arr) pair WITH OFFSET
    WHERE trim(pair) != 'Res -> Res'
    ORDER BY offset LIMIT 1
  ) AS next_word
FROM your_table, UNNEST([struct(regexp_extract_all(path, r' Res -> \w+') as arr)])

Note that the examples above are simplified and may not cover all possible use cases. You should consult the official BigQuery documentation for more information on how to extract values from arrays.

Conclusion

In conclusion, extracting the next value after “Res” in an array is a common task when working with nested structures or hierarchies in BigQuery. There are two approaches to solve this problem: using offset and trim functions, and using regular expressions and trim functions. The second approach provides more flexibility and accuracy than the first approach. We hope that this article has provided you with a clear understanding of how to extract values from arrays in BigQuery.

Last modified on 2024-05-18