How do you CROSS JOIN UNNEST a JSON array in Presto?

Let’s take a look at how you can cross join unnest a JSON array by using Presto joins. Here’s an example using test results data in json form as input, from which we compute the average score per student.  We use the WITH clause to define a common table expression (CTE) named example with a column alias name of data. The VALUES function returns a table rowset. 

WITH example(data) as 
(
    VALUES
    (json '{"result":[{"name":"Jarret","score":"90"},{"name":"Blanche","score":"95"}]}'),
    (json '{"result":[{"name":"Blanche","score":"76"},{"name":"Jarret","score":"88"}]}')
)
SELECT n.name as "Student Name", avg(n.score) as "Average Score"
FROM example
CROSS JOIN
    UNNEST ( 
        CAST (JSON_EXTRACT(data, '$.result')
        as ARRAY(ROW(name VARCHAR, score INTEGER )))
    ) as n
--WHERE n.name='Jarret'
GROUP BY n.name;

Student Name | Average Score 
-------------+---------------
 Jarret      |          89.0 
 Blanche     |          85.5 
(2 rows)

The UNNEST function takes an array within a column of a single row and returns the elements of the array as multiple rows.

CAST converts the JSON type to an ARRAY type which UNNEST requires.

JSON_EXTRACT uses a jsonPath expression to return the array value of the result key in the data.

The UNNEST approach is similar to Hive’s explode function.

This code sample and description should help when you need to execute a cross join to unnest a JSON array. 

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