SQL Query keeps returning multiple rows

Question

I wrote this SQL Query to get the remaining benefits for a particular patient, expecting it to return a single row since the patient id is unique, but it keeps returning multiple rows. How do I fix this? DBMS is MySql

 select Plan.pBenefitMax - sum(Transaction.employerTotal) as RemainingBenefit
 from Patient
 inner join Plan on Patient.pPlanId = Plan.planId
 inner join Transaction on Patient.patientId = Transaction.tPatientId
  where Patient.patientId = 1
 Group by Plan.pBenefitMax;

Answer 1

If this does not get a single row, you either have duplicate PatientId values or the PatientId is linked to more than one Plan.

  SELECT Plan.pBenefitMax - 
         (SELECT SUM(`Transaction`.employerTotal) 
            FROM `Transaction` 
           WHERE `Transaction`.tPatientId = Patient.patientId) 
         AS RemainingBenefit
    FROM Patient
    JOIN Plan ON Patient.pPlanId = Plan.planId
   WHERE Patient.patientId = 1;