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Asked: 2024-09-25T14:29:43+05:30 In:

How can I determine the length of a JSON array using Flink SQL?

So, I’ve been diving into Flink SQL lately, and I hit a bit of a snag with something that seems like it should be simple but has me scratching my head. I’m trying to figure out how to determine the length of a JSON array within a table. You know, I thought it would be straightforward—just pull up the array and count the elements—but the syntax is just throwing me for a loop.

Here’s the deal: I have a dataset where one of the columns contains JSON objects, and within those objects, there’s an array. It holds a variable number of elements, and I want to be able to create a query that gives me the length of that array for each row. I’ve scoured documentation and various forums, but I can’t seem to nail down the right function or method in Flink SQL that would give me what I need.

For example, if I have a JSON column that looks something like this:

“`json
{“id”: 1, “values”: [10, 20, 30]}
“`

I want my result to show that for this row, the length of the “values” array is 3. But when I try to write a query, I either get errors or end up with bizarre results that don’t make any sense.

Has anyone out there managed to successfully get the length of a JSON array in Flink SQL? What functions are you using, or is there a specific query structure that you found works best? Even just sharing a small snippet of how you approached this would be super helpful! I’m all sorts of confused here, and switching between different JSON handling methods just isn’t working out for me. Would really appreciate any insights or examples you could share, so I can finally get this length figured out and move on with my project. Thanks!

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    2 Answers


    1. To determine the length of a JSON array in a Flink SQL table, you can leverage the built-in function `JSON_LENGTH`, which is quite useful for such use cases. Assuming your dataset contains a column named `json_column` where the JSON objects reside, you would structure your SQL query as follows:
       

      SELECT JSON_LENGTH(json_column, '$.values') AS values_length
FROM your_table;

      This query utilizes the `JSON_LENGTH` function to extract the length of the array located at the given JSON path `$.values`. Make sure to replace `your_table` with the actual name of your table. This should yield a result with the length of the “values” array for each row in your dataset.

      If you encounter any issues, ensure that your JSON data is properly formatted and that the key path accurately reflects your data structure. Another common function you might find useful is `JSON_VALUE`, which can also help you to manipulate and retrieve specific elements from JSON objects. For example, you might want to check snippets of your JSON using `JSON_VALUE` to ensure the data structure is as expected, thus helping debug any discrepancies you might face while querying.




    2. Flink SQL Help

      Determining Length of JSON Array in Flink SQL

      Looks like you’re in a bit of a bind with that JSON array length! It can be a little tricky at first, but let’s see if we can untangle this together.

      In Flink SQL, you can use the JSON_VALUE function to extract the array from your JSON column, and then you can get the length of that array using the ARRAY_LENGTH function. Here’s a simple example query to get you started:

              SELECT 
            id, 
            ARRAY_LENGTH(JSON_VALUE(your_json_column, '$.values')) AS array_length
        FROM 
            your_table;

      Replace your_json_column with the actual name of your JSON column and your_table with the name of your table.

      So in your case, you might have a query that looks something like this:

              SELECT 
            JSON_VALUE(data, '$.id') AS id, 
            ARRAY_LENGTH(JSON_VALUE(data, '$.values')) AS values_length
        FROM 
            my_json_table;

      Assuming your column is called data and the table is my_json_table, this should give you the length of the “values” array for each row.

      If you run into issues, double-check your syntax, and make sure your JSON path is correct. It can be a bit finicky, so don’t lose hope!

      Hope that helps you move forward with your project!


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