CREATE TABLE INVESTORS (investor_id VARCHAR2)
CREATE TABLE TRANSACTIONS (investor_id VARCHAR2)

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" "T1" JOIN "TRANSACTIONS" "T2" ON "T1"."investor_id" = "T2"."investor_id" GROUP BY "T2"."investor_id" HAVING COUNT(*) >= 2