这段sql代码什么意思？解决办法

INSERT INTO ProductCode                      (ProductID, SupplierID, ProductCode, ConvertRate)SELECT     PorderImport.MEDICAL_CODE, PorderImport.SALER_CODE, PorderImport.MEDICAL_CODE AS Expr2, 1 AS Expr1FROM         ProductCode AS ProductCode_1 RIGHT OUTER JOIN                      PorderImport INNER JOIN                      Supplier ON PorderImport.SALER_CODE = Supplier.SupplierID INNER JOIN                      Product ON PorderImport.MEDICAL_CODE = Product.ProductID ON ProductCode_1.ProductID = PorderImport.MEDICAL_CODE AND                       ProductCode_1.SupplierID = PorderImport.SALER_CODEWHERE     (ProductCode_1.ProductID IS NULL) AND (Supplier.Discontinued = 0) AND (Supplier.Status = N'2') AND (Supplier.EPS = 1)GROUP BY PorderImport.MEDICAL_CODE, PorderImport.SALER_CODE