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  • Optimizing BigQuery Google Analytics Sample: Handling repetitive nested elements with CASE WHEN logic

    Posted by Jayden on 10 March 2023 at 4:30 am

    I’m getting a bit hung up on this part of our SQL query:

    CASE WHEN type=’EVENT’ THEN product.v2ProductName[SAFE_OFFSET(1)] ELSE NULL END.

    When we exclude this bit, everything seems to be smooth sailing. However, once we add it, we run into trouble. I’ve tried various fixes, like unnesting within the case statement (even though “orig” is already unnested in our temporary table), or using OFFSET and SAFE_OFFSET with or without UNNEST, but no luck yet.

    Just as a side note, “product” doesn’t need to be unnested (it’s the same deal for “page” and “eventInfo”). Here’s our current version of the SQL query:

    (Then proceed to mention the SQL query.)

    As for the output, I’m hoping for a frame with these fields: visitor_id, last_visit, last_hit, type, page, product, action, and label. And this should be generated for every user who hasn’t made a purchase.

    Just FYI, the hits.product.v2ProductName field is nested and repetitive. However, when the event type is ‘EVENT’, it’s not repetitive (unlike when the event type is social or page). As such, theoretically, I shouldn’t have to use OFFSET to select a particular element from the list, although I’ve tried with and without it all the same. If you have any suggestions, I’d be grateful. Thanks!

    Mia replied 11 months, 2 weeks ago 3 Members · 2 Replies
  • 2 Replies
  • Amelia

    16 June 2023 at 7:18 pm

    Your SQL query has an issue with referencing nested and possibly repetitive fields in a SAFE_OFFSET function when the type condition is met. Since the v2ProductName field is only non-repetitive in ‘EVENT’ instances, it is plausible the function might return null, non-existent, or throw an error for other event types.

    One option could be to flatten or UNNEST the ‘product’ field to a selectable structure before this case statement. This way, you can select the v2ProductName or its conditionally offset variant in all circumstances.

    Regarding the nature of your desired output, you’d presumably want to encapsulate the logic in an encapsulating select statement or the FROM clause that filters only non-purchasing visitors. You could ensure this by using an outer join or where-not-exists clause for users with corresponding purchase records.

    Also, understand that SQL is set-based logic and does not work well with ‘lists,’ per se. The whole concept of OFFSET means nothing in strict relational database design, and therefore, using functions like SAFE_OFFSET might lead to unpredictable behavior that does not adhere well with the raw logic of SQL. Consequently, having repetitive fields and the requirement to select a specific instance within these is generally a symptom of non-optimal table structure for a relational database context.

    In conclusion: restructure your data so that it is “clean” in a SQL context. Preferably each entity and attribute should exist only once, and foreign keys should be used to deal with one-to-many or many-to-many relationships.

  • Mia

    18 June 2023 at 2:55 pm

    It appears that you’re dealing with a complex nested data structure in SQL, which can get quite tricky to handle. The issue you’re facing seems to revolve around the CASE WHEN statement in conjunction with SAFE_OFFSET. SAFE_OFFSET is normally used to extract a value from an array without running into out-of-bound errors. However, in your case, the product.v2ProductName doesn’t require unnesting or offsetting when the type is ‘EVENT’.

    Since ‘type’ and ‘product’ don’t need to be unnested, your CASE statement might be failing because it’s trying to SAFE_OFFSET on a non-array value or NULL.

    Alternatively, the issue might be stemming from how SQL is handling the nesting of your hits.product.v2ProductName field. If the structure is not consistent across all your data (specifically, if ‘EVENT’ type does not always have a non-repetitive array for this field), it could cause errors.

    Try revisiting how you’re handling your nested product data, particularly when the type is ‘EVENT’. Assure that your SQL query is able to handle these conditions correctly. If the problem persist, you may need to change the structure of your data for better consistency, or alter your query to handle this inconsistency.

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