每个分组的SQL计数

Question

So I am writing a SQL statement for the following. List the count of patients for each insurance code.

here is my SQL Statement.

SELECT INSURANCE.INS_ID, PATIENT.MPI_LastName
FROM PHYSICIAN 
     INNER JOIN 
           (PATIENT INNER JOIN 
                    (INSURANCE INNER JOIN VISIT 
                       ON INSURANCE.INS_ID = VISIT.VIS_InsuranceCode
                    ) 
             ON PATIENT.MPI_NUMBER = VISIT.VIS_MPINum
           ) 
           ON PHYSICIAN.PHY_ID = VISIT.VIS_Phys
ORDER BY INSURANCE.INS_ID;

I do not know how to write a SQL to count the Patients for each insurance code. without duplicate records being counted.

Answer 1

SELECT INSURANCE.INS_ID, count(distinct PATIENT.MPI_LastName) as count 
  FROM VISIT    
  JOIN PATIENT  
        ON PATIENT.MPI_NUMBER = VISIT.VIS_MPINum
  JOIN INSURANCE 
        ON INSURANCE.INS_ID   = VISIT.VIS_InsuranceCode 
 GROUP BY INSURANCE.INS_ID


  SELECT VISIT.VIS_InsuranceCode, count(distinct PATIENT.MPI_LastName) as count 
  FROM VISIT    
  JOIN PATIENT  
        ON PATIENT.MPI_NUMBER = VISIT.VIS_MPINum
 GROUP BY VISIT.VIS_InsuranceCode