How to call the External URL to get the data in SharePoint REST API

Hi,

I have created the web api URL to get the data from SQL database. Now i want to call that api in SharePoint hosted app REST api call. If i call that url it will get the data and show it in SharePoint. I have called this using the normal Ajax call but it throws an error. 

$.ajax({
        url: "../_api/SP.WebProxy.invoke",
        type: "POST",
        data: JSON.stringify(
            {
                "requestInfo": {
                    "__metadata": { "type": "SP.WebRequestInfo" },
                    "Url": externalUrl,
                    "Method": "GET",
                    "Headers": {
                        "results": [{
                            "__metadata": { "type": "SP.KeyValue" },
                            "Key": "Accept",
                            "Value": "application/json;odata=verbose",
                            "ValueType": "Edm.String"
                        }]
                    }
                }
            }),
        headers: {
            "Accept": "application/json;odata=verbose",
            "Content-Type": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },

I have tried this way. but it shows the forbidden error. Can anyone please suggest the solution?

Thanks in advance.