从Presto的JSON列中选择特定值

Question

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

Answer 1

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

Answer 2

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)