CODEWITHSUNDEEP
sqldatabasepresto
Hellboy
Hellboy

Reputation: 1239

Select particular values from JSON column in Presto

I have a JSON column points of type VARCHAR in a table which I want to parse in Presto. For example: 

points = {"0": 0.2, "1": 1.2, "2": 0.5, "15": 1.2, "20": 0.7}

I want to select only the values for keys "0", "2" and "20". How do I use the UNNEST functionality of Presto to get them. What I've done till now is:

select t.value from myTable CROSS JOIN UNNEST(points) AS t(key, value) limit 1

But this gives this error:

Cannot unnest type: varchar

Update:

I ran the following query and got the result but it is returning one random key-value pair from the JSON whereas I need specific keys.

select key, value from myTable CROSS JOIN UNNEST(SPLIT_TO_MAP(points, ',', ':')) AS t(key, value) limit 1

Upvotes: 1

Views: 5480

Answers (2)

FreePeter
FreePeter

Reputation: 772

You can unnest an Array or Map. So you first need to convert the JSON string into a MAP:

CAST(json_parse(str) AS MAP<BIGINT, DOUBLE>)

Here is an example:

presto> select tt.value
     -> from (VALUES '{"0": 0.2, "1": 1.2, "2": 0.5, "15": 1.2, "20": 0.7}') as t(json)
     -> CROSS JOIN UNNEST(CAST(json_parse(json) AS MAP<BIGINT, DOUBLE>)) AS tt(key, value)
     -> ;
 value
-------
   0.2
   1.2
   1.2
   0.5
   0.7
(5 rows)

Upvotes: 1

kjones
kjones

Reputation: 1423

You may need to cast to json datatype first according to these docs: enter link description here

UNNEST(CAST(points AS JSON))

Full query:

select t.value from myTable
CROSS JOIN UNNEST(CAST(points AS JSON)) AS t(key, value) limit 1

Upvotes: 0

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