Although the Hive API generally allows you to retrieve information from the blockchain, there is no API call to easily know who has delegated RCs to whom.
Fortunately, HiveSQL allows you to retrieve related information thanks to the TxCustoms table which includes all creation, modification or removal of RC delegation operations carried out by users.
However, the main difficulty comes from the fact that you have to filter custom_json operations, parse the JSON payload and perform computations with the retrieved operation to get the state of the delegation between accounts.
Due to all the technical reasons mentioned above, retrieving an accurate list of delegators or delegatees has become much more complex and resources consuming.
The non-consensus state table RCDelegations allows you to retrieve delegations data quickly and easily. It contains the list of all active delegations with the following info:
The account delegating Hive Power
The account receiving Hive Power delegation
The amount of RCs delegated