How to get elements from Json array in PostgreSQL

Question

I have searched quite much on this and still unanswerable. I'm using PostgreSQL. Column name is "sections" and column type is json[] in below example.

My column looks like this in database:

sections
[{"name"      : "section1",
  "attributes": [{"attrkey1": "value1",
                  "attrkey2": "value2"},

                 {"attrkey3": "value3",
                  "attrkey4": "value4"}]
 },
 {"name"      : "section2",
  "attributes": [{"attrkey3": "value5",
                  "attrkey6": "value6"},

                 {"attrkey1": "value7",
                  "attrkey8": "value8"}]
 }]

It's json array and I want to get "attrkey3" in my result. For getting particular key from Json, I can use json_extract_path_text(json_column, 'json_property') which is working perfectly fine. But I have no idea how to get some property from json[].

If I talk about above example, I want to get value of property "attrkey2" to be shown in my result. I know it's an array so it might work differently than usual, e.g. all the values of my array would act as a different row so I might have to write subquery but no idea how to do it.

Also, I can't write index statically and get property of the json element from some particular index. My query will be generated dynamically so I would never know how many elements are inside json array.

I saw some static examples but don't know how to implement it in my case. Can someone tell me how to do this in query?

Answer 1

I'm not sure you have a json[] (PostgreSQL array of json values) typed column, or a json typed column, which appears to be a JSON array (like in your example).

Either case, you need to expand your array before querying. In case of json[], you need to use unnest(anyarray); in case of JSON arrays in a json typed column, you need to use json_array_elements(json) (and LATERAL joins -- they are implicit in my examples):

select     t.id,
           each_section ->> 'name' section_name,
           each_attribute ->> 'attrkey3' attrkey3
from       t
cross join unnest(array_of_json) each_section
cross join json_array_elements(each_section -> 'attributes') each_attribute
where      (each_attribute -> 'attrkey3') is not null; 
-- use "where each_attribute ? 'attrkey3'" in case of jsonb


select     t.id,
           each_section ->> 'name' section_name,
           each_attribute ->> 'attrkey3' attrkey3
from       t
cross join json_array_elements(json_array) each_section
cross join json_array_elements(each_section -> 'attributes') each_attribute
where      (each_attribute -> 'attrkey3') is not null;

SQLFiddle

Unfortunately, you cannot use any index with your data. You need to fix your schema first, in order to do that.

Answer 2

If you wish to access a single element then use json_array -> index

For example, if you have json_arr=[1,2,3] then json_array -> 0 will return 1

Answer 3

And also, if there was a key value map data in array:

select each_data -> 'value' as value3 
from t cross join jsonb_array_elements(t.sections -> 'attributes') each_attribute 
where each_attribute -> 'key' = '"attrkey3"'

I am mentioning this because the great answer also provided a perfect solution for my case. By the way, also be aware of jsonb_array.. method for jsonb type attribute.