How to get elements with a unique number from a json array in PostgreSQL?

问题

I have a table bank_accounts:

    Column     |         Type          |                                Modifiers                                | Storage  | Stats target | Description 
---------------+-----------------------+-------------------------------------------------------------------------+----------+--------------+-------------
 id            | integer               | not null default nextval('bank_accounts_id_seq'::regclass)              | plain    |              | 
 name          | character varying(50) |                                                                         | extended |              | 
 bank_accounts | jsonb                 | not null                                                                | extended |              |

And it has some JSON in the jsonb column:

 id | name  |                              bank_accounts                               
----+-------+--------------------------------------------------------------------------
  1 | test1 | [{"name": "acct1", "balance": -500}, {"name": "acct2", "balance": -300}]

And I am using jsonb_array_elements to get a list of the accounts for one user:

select jsonb_array_elements(bank_accounts)->>'name' as name, jsonb_array_elements(bank_accounts)->>'balance' as balance from bank_accounts;
 name  | balance 
-------+---------
 acct1 | -500
 acct2 | -300

That's all great. But how do I get each row to have a unique id? I'd like to map each row to a hibernate object, but I'm having trouble doing that because I can't find a way to get each row to have a unique id.

回答1:

Try a different, clean approach with JOIN LATERAL:

select b.id, t.rn
     , t.account->>'name' AS name
     , t.account->>'balance' AS balance
FROM   bank_accounts b
LEFT   JOIN LATERAL jsonb_array_elements(b.bank_accounts)
                    WITH ORDINALITY AS t (account, rn) ON true;

If you don't care for rows with empty or null values in bank_accounts, use a simpler CROSS JOIN:

select b.id, t.rn
     , t.account->>'name' AS name
     , t.account->>'balance' AS balance
FROM   bank_accounts b
     , jsonb_array_elements(b.bank_accounts) WITH ORDINALITY AS t (account, rn);

The key element for your problem is WITH ORDINALITY which produces row numbers on the fly for set-returning functions. It was introduced with Postgres 9.4 - works for you, jsonb was also introduced with 9.4.

Those are unique per underlying row. To be unique across the whole table, add the id of the underlying table.

Details for WITH ORDINALITY: