jsonb, PostgreSQL - rextester

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create table js(id int, msg jsonb);
insert into js values
(1, '{"root":[[
            {"cid":"CID1","Display":"User One","FName":"User","LName":"One"},
            {"cid":"CID1","Display":"User Two","FName":"User","LName":"Two"},
            {"cid":"CID1","Display":"User Three","FName":"User","LName":"Three"},
            {"cid":"CID1","Display":"User One","FName":"User","LName":"One"},
            {"cid":"CID1","Display":"User Two","FName":"User","LName":"Two"},
            {"cid":"CID1","Display":"User Three","FName":"User","LName":"Three"}

],
            [
            {"cid":"CID2","Display":"User One","FName":"User","LName":"One"},
            {"cid":"CID2","Display":"User Two","FName":"User","LName":"Two"},
            {"cid":"CID2","Display":"User Three","FName":"User","LName":"Three"},
            {"cid":"CID2","Display":"User One","FName":"User","LName":"One"},
            {"cid":"CID2","Display":"User Two","FName":"User","LName":"Two"},
            {"cid":"CID2","Display":"User Three","FName":"User","LName":"Three"}    
            ]]}'::jsonb);

-- set of rows
                
with aa as
(
    select jsonb_array_elements(msg->'root') as el
    from js
)
select jsonb_array_elements(el)->>'cid' as cid,
       jsonb_array_elements(el)->>'Display' as Display,
       jsonb_array_elements(el)->>'FName' as FName,
       jsonb_array_elements(el)->>'LName' as LName         
from aa;

-- Returns 3rd element of 2nd array
                
select (ARRAY(select * from jsonb_array_elements(A1.Elements)))[3] Elem
from
    (select (ARRAY(select * from jsonb_array_elements(msg->'root')))[2] Elements
    from js) A1
;


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