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Asked: 2024-09-25T14:08:58+05:30 In:

How can I convert the output of a SQL query in Trino or Presto into an array using the format `CAST(b AS ARRAY)`? I am looking for a solution to structure data into an array and would appreciate any guidance on the appropriate syntax or methods to achieve this.

I’ve been diving into some queries using Trino, and I’m a bit stumped on how to format my output into an array. I want to convert the result of a SQL query into an array using something like `CAST(b AS ARRAY)`, but I’m not entirely sure how to pull that off.

Here’s the situation: I’m working with a dataset that includes several columns, specifically `a`, `b`, and `c`. I can run a query that retrieves these columns without too much trouble, but the challenge comes when I try to aggregate the results into a structured array format. I think this could be super useful for making my data more concise and manageable, especially since I want to pass this array to another component of my application.

I’ve read through some documentation, and while I can see that casting in Trino is pretty straightforward, I’m a little confused about how to structure this when I’m working with row types. For instance, if my initial query gives me something like:

“`
SELECT a, b, c
FROM my_table;
“`

What would I need to do next to convert this result into an array of rows?

Are there specific functions or syntax I should be looking at? Should I use a GROUP BY clause if I want to consolidate multiple rows into one array? I want to make sure I understand the nuances of how Trino handles these transformations, especially since SQL can sometimes be tricky with arrays.

Any examples or snippets you could share would be awesome! I’ve been trying some variations, but they mostly end up erroring out or just not producing the results I need. Thanks in advance for any tips or guidance on this!

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    2 Answers

    1. It sounds like you’re trying to convert the results of your SQL query into an array of rows in Trino. No worries, I can help you out with that!

      First, you’re on the right track with wanting to use something like CAST(b AS ARRAY). The idea is to use the ARRAY_AGG function, which is super helpful for aggregating multiple rows into a single array.

      Your initial query is:

              SELECT a, b, c 
        FROM my_table;

      To convert this into an array format, you can do something like this:

              SELECT 
            ARRAY_AGG(ROW(a, b, c)) AS my_array
        FROM 
            my_table;

      This will give you a single row containing an array of rows structured as specified. Each element in the array will be a row with elements `a`, `b`, and `c`.

      If you want to group your results by a specific column (like `a` for example), you can add a GROUP BY clause:

              SELECT 
            a, 
            ARRAY_AGG(ROW(a, b, c)) AS my_array
        FROM 
            my_table
        GROUP BY 
            a;

      Just remember that the types of `a`, `b`, and `c` need to be compatible with the row type you’re creating. Make sure you’ve got those right! If you encounter any errors, it’s often helpful to check the data types or the syntax.

      Messing around with this can be a bit tricky at first, but once you get the hang of it, it’s really powerful! Let me know if you have any other questions!


    2. To convert your SQL query results into an array of rows in Trino, you can utilize the `ARRAY_AGG` function in combination with a structured type. Assuming your query retrieves the columns `a`, `b`, and `c` from `my_table`, you would modify your query to form an array of row types. The syntax would look like this:

                  SELECT ARRAY_AGG(ROW(a, b, c)) AS array_of_rows
            FROM my_table;

      This approach uses the `ARRAY_AGG` function to collect all the rows produced by the query into a single array. The `ROW(a, b, c)` constructs a row type for each entry, effectively packaging the columns together. If you need to consolidate multiple rows into one array, make sure to include a `GROUP BY` clause to group by the desired criteria, such as a specific identifier or date. For example:

                  SELECT some_id, ARRAY_AGG(ROW(a, b, c)) AS array_of_rows
            FROM my_table
            GROUP BY some_id;

      This will aggregate the rows into an array grouped by `some_id`, ensuring that the output is concise and manageable for further application processing.


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