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Discover how to write effective UNNEST queries in BigQuery to return a minimum of 10 unique rows by adjusting your LIMIT clause.
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How to Ensure a Minimum of 10 Unique Rows with UNNEST in BigQuery

If you've ever found yourself dealing with nested data in BigQuery, you might have encountered challenges when trying to retrieve a specific number of unique records. A common issue arises when using the UNNEST function along with a LIMIT clause, where the results can often return duplicates due to the nature of the data relationships. In this guide, we will walk through how to manage this problem specifically for ensuring a minimum of 10 unique rows in your query results.

Understanding the Problem

You are working with a dataset that contains information about food items, including their variations. Here’s a brief overview of the relevant data structure:

Food ID: Unique identifier for food items

Basket ID: Identifier for the basket each food item belongs to

Food Type: Type of food (e.g., JUNK, PIZZA, TOAST)

Price: Cost of the food item

Printed: Boolean flag for whether the item has been printed

Variations: A repeated record that can contain multiple variations for each food item

By using a query that includes UNNEST, you aim to expand the variations of each food item. However, adding LIMIT directly after the SELECT statement can lead to losing important data entries, as it could limit your results before they are fully processed.

Sample Data

Here’s a brief look at the sample data that we are dealing with:

[[See Video to Reveal this Text or Code Snippet]]

The Challenge

When running the following query:

[[See Video to Reveal this Text or Code Snippet]]

You noticed that a row with basket.id of uiwka was missing from the results. The LIMIT clause, when applied early, restricts the complete set of potential unique rows before any deduplication can occur.

The Solution: Adjusting the Query

To ensure you obtain a minimum of 10 unique records, you should adjust your query to limit the original dataset before it gets unnested. Here’s how to do that:

Revised Query Structure

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Query

Subquery with LIMIT: By placing the LIMIT inside a subquery, you first limit the number of unique basket.id entries. This ensures you only retrieve a maximum of 10 distinct records.

JOIN with UNNEST: After limiting the dataset, using LEFT JOIN UNNEST(variations) AS v allows you to fetch all relevant variation details without inadvertently cutting off unique basket IDs.

Resulting Output: This approach guarantees that you are pulling data correctly, increasing the chances that you will include all necessary variations while keeping at least 10 unique basket ID entries.

Conclusion

By strategically structuring your BigQuery queries with a subquery handling the limit, you can efficiently return a set of data that meets your criteria for uniqueness and comprehensiveness. This method proves invaluable for managing nested datasets where duplicates can easily skew the results.

Now, you can confidently execute your queries and ensure that you always get a minimum number of unique records, enhancing the accuracy of your data retrieval in BigQuery.