Extract elements from JSON array and return them as concatenated string - postgresql

Question

A PostgreSQL 10 table contains JSON data like (here an SQL Fiddle):
[
{
"col": 7,
"row": 12,
"value": 3,
"letter": "A"
},
{
"col": 8,
"row": 12,
"value": 10,
"letter": "B"
},
{
"col": 9,
"row": 12,
"value": 1,
"letter": "C"
},
{
"col": 10,
"row": 12,
"value": 2,
"letter": "D"
}
]
How to extract only the "letter" values and concatenate them to a string like
ABCD
I suppose at the end I should use the ARRAY_TO_STRING function, but which JSON function to use for extracting the "letter" values to an array?
UPDATE:
Got a tip at the very helpful PostgreSQL mailing list too:
SELECT string_agg(x->>'letter','') FROM json_array_elements(
'[{"col": 7, "row": 12, "value": 3, "letter": "A"}, {"col": 8, "row": 12, "value": 10, "letter": "B"}, {"col": 9, "row": 12, "value": 1, "letter": "C"}, {"col": 10, "row": 12, "value": 2, "letter": "D"}]'::json
) x;

Answer 1

Use jsonb_array_elements() and string_agg():
with my_table(json_data) as (
values(
'[
{
"col": 7,
"row": 12,
"value": 3,
"letter": "A"
},
{
"col": 8,
"row": 12,
"value": 10,
"letter": "B"
},
{
"col": 9,
"row": 12,
"value": 1,
"letter": "C"
},
{
"col": 10,
"row": 12,
"value": 2,
"letter": "D"
}
]'::jsonb)
)
select string_agg(value->>'letter', '')
from my_table
cross join lateral jsonb_array_elements(json_data)
string_agg
------------
ABCD
(1 row)