Query List with REST API in app web from host web on different domain

I am successful in getting data from host web to app web but the problem i am facing with is querying the API.

This is i am trying to do but getting error..

    var getTaskByUser = function (userID) {
            var hostweburl =
                decodeURIComponent(
                    getQueryStringParameter("SPHostUrl")
            );
           var  appweburl =
                decodeURIComponent(
                    getQueryStringParameter("SPAppWebUrl")
            );
            var scriptbase = hostweburl + "/_layouts/15/";

            // Load the js files and continue to the successHandler
            $.getScript(scriptbase + "SP.RequestExecutor.js", execCrossDomainRequest);
            function execCrossDomainRequest() {
            // executor: The RequestExecutor object
            // Initialize the RequestExecutor with the add-in web URL.
            var executor = new SP.RequestExecutor(appweburl);
            executor.executeAsync(
    {
        url:
            appweburl +
            "/_api/SP.AppContextSite(@target)/web/lists/GetByTitle('Asana2')/Items?$select=Title,Category?@target='" +
            hostweburl + "'",
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
        success: successHandler,
        error: errorHandler
    }
);

ERROR:

 GET https://niisar-3b7c10c2234504.sharepoint.com/Development/SharePointApp1/_ap…ect=Title,Category?@target=%27https://niisar.sharepoint.com/Development%27 
400 (Bad Request)

"{"error":{"code":"-1, Microsoft.SharePoint.Client.InvalidClientQueryException","message":{"lang":"en-US","value":"The expression \"Title,Category?@target='https://niisar.sharepoint.com/Development'\" is not valid."}}}"

Need help. I am new to sharepoint