• A percentage is just a decimal type with X integers and Y decimals.
    Most cases will store percentages as decimal (9, 4)  but this is always going to depend on the business rules.

    As for storing the symbol there is no data type for it other than char datatypes - but this should not be used on this case.
    And the reason why you see .15 being stored is because Excel stores it as a decimal value internally. Displaying 15% is just a function of the interface, not of the database.