Export maximum record not working

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Could anyone help me understand how to export a maximum record for an employee who has received a review rating?  An employee has multiple review ratings, some with and some without review ratings.  I only want to export the employes last review rating that is not null.  I see I that I can find out the latest rating that is not null by looking at the ManagerReview,OverallScore,RatingValue,NumericValue.  But in my query, it is still returning multple records for employee.