Show the ids of the investors who have at least two transactions.
SELECT T2.investor_id FROM INVESTORS AS T1 JOIN TRANSACTIONS AS T2 ON T1.investor_id  =  T2.investor_id GROUP BY T2.investor_id HAVING COUNT(*)  >=  2
SELECT T2.investor_id FROM INVESTORS AS T1 JOIN TRANSACTIONS AS T2 ON T1.investor_id  =  T2.investor_id GROUP BY T2.investor_id HAVING COUNT(*)  >=  2
[{'investor_id': 2}, {'investor_id': 3}, {'investor_id': 18}, {'investor_id': 19}]