Check for duplicate values for a specific JSON key - json

Question

I have the following JSON records stored in a container
{"memberId":"123","city":"New York"}
{"memberId":"234","city":"Chicago"}
{"memberId":"345","city":"San Francisco"}
{"memberId":"123","city":"New York"}
{"memberId":"345","city":"San Francisco"}
I am looking to check if there is any duplication of the memberId - ideally return a true/false and then also return the duplicated values.
Desired Output:
true
123
345

Answer 1

Here's an efficient approach using inputs. It requires invoking jq with the -n command-line option. The idea is to create a dictionary that keeps count of each memberId string value.
The dictionary can be created as follows:
reduce (inputs|.memberId|tostring) as $id ({}; .[$id] += 1)
Thus, to produce a true/false indicator, followed by the duplicates if any, you could write:
reduce (inputs|.memberId|tostring) as $id ({}; .[$id] += 1)
| to_entries
| map(select(.value > 1))
| (length > 0), .[].key
(If all the .memberId values are known to be strings, then of course the call to tostring can be dropped. Conversely, if .memberId is both string and integer-valued, then the above program won't differentiate between occurrences of 1 and "1", for example.)
bow
The aforementioned dictionary is sometimes called a "bag of words" (https://en.wikipedia.org/wiki/Bag-of-words_model). This leads to the generic function:
def bow(stream):
reduce stream as $word ({}; .[($word|tostring)] += 1);
The solution can now be written more concisely:
bow(inputs.memberId)
| to_entries
| map(select(.value > 1))
| (length > 0), .[].key
For just the values which have duplicates, one could write the more efficient query:
bow(inputs.memberId)
| keys_unsorted[] as $k
| select(.[$k] > 1)
| $k