The ORA-01427 is a straightforward query error in Oracle. The cause of the error is a subquery returning more than one row of information.
Try the following SQL statement:
SELECT *
FROM employers
WHERE client_id = (SELECT client_id
FROM clients
WHERE clients_name = ‘Walmart’) ;
If more than one instance in the client’s table of the clients_name ‘Walmart’ existed, the following would be prompted:
ORA-01427: single-row subquery returns more than one row