Query a feature layer (SQL)

Learn how to execute a SQL query to return features from a feature layer based on spatial and attribute criteria.

A feature layer can contain a large number of features stored in ArcGIS. You can query a layer to access a subset of its features using any combination of spatial and attribute criteria. You can control whether or not each feature's geometry is returned, as well as which attributes are included in the results. Queries allow you to return a well-defined subset of your hosted data for analysis or display in your app.

In this tutorial, you'll write code to perform SQL queries that return a subset of features in the LA County Parcel feature layer (containing over 2.4 million features). Features that meet the query criteria are selected in the map.

Prerequisites

Steps

Open a Visual Studio solution

1. To start the tutorial, complete the Display a map tutorial or download and unzip the solution.

2. Open the .sln file in Visual Studio.

3. If you downloaded the solution, get an access token and set the API key.
   Instructions for setting the API key

   An API Key gives your app access to secure resources used in this tutorial.

   1. Go to the Create an API key tutorial to obtain a new API key access token using your ArcGIS Location Platform or ArcGIS Online account. Ensure that the following privilege is enabled: Location services > Basemaps > Basemap styles service. Copy the access token as it will be used in the next step.

   2. In Visual Studio, in the Solution Explorer, click App.xaml.cs.

   3. In the App class, add an override for the OnStartup() function to set the ApiKey property on ArcGISRuntimeEnvironment.

      App.xaml.cs

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          public partial class App : Application
          {
      
              protected override void OnStartup(StartupEventArgs e)
              {
                  base.OnStartup(e);
                  Esri.ArcGISRuntime.ArcGISRuntimeEnvironment.ApiKey = "YOUR_ACCESS_TOKEN";
              }
      
          }
      }

Update the tutorial name used in the project (optional)

The Visual Studio solution, project, and the namespace for all classes currently use the name DisplayAMap. Follow the steps below if you prefer the name to reflect the current tutorial. These steps are not required, your code will still work if you keep the original name.

Steps

Add UI for selecting a predefined query expression

To make performing a query on a feature layer more flexible, add a ComboBox control to present a list of predefined attribute queries for the parcels dataset. While editing this file, also modify the MapView control to bind its SelectionProperties to a property in the view model (so you can set the color used to display selected parcels).

1. In the Visual Studio > Solution Explorer, double-click the MainWindow.xaml file to open it.

2. Add XAML that defines a ComboBox control and its behavior.

   • The control is positioned above the map view in the upper-left corner.
   • A list of ComboBoxItems shows predefined expression choices.
   • The first combo box item instructs the user to choose an expression that will be selected when the app initializes.
   • Changes in the selected expression will be handled by the QueryComboBox_SelectionChanged event handler.

   Also modify the MapView control to bind the SelectionProperties property.

   MainWindow.xaml

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       <Grid>
   
           <esri:MapView x:Name="MainMapView"
                         Map="{Binding Map, Source={StaticResource MapViewModel}}"
                         SelectionProperties="{Binding SelectionProps, Source={StaticResource MapViewModel}}"/>
   
           <ComboBox x:Name="QueryComboBox"
                     SelectionChanged="QueryComboBox_SelectionChanged"
                     SelectedIndex="0"
                     SelectedValuePath="Content"
                     HorizontalAlignment="Left" VerticalAlignment="Top"
                     Margin="20" Padding="30,10">
               <ComboBoxItem>Choose a SQL where clause</ComboBoxItem>
               <ComboBoxItem>UseType = 'Government'</ComboBoxItem>
               <ComboBoxItem>UseType = 'Residential'</ComboBoxItem>
               <ComboBoxItem>UseType = 'Irrigated Farm'</ComboBoxItem>
               <ComboBoxItem>TaxRateArea = 10853</ComboBoxItem>
               <ComboBoxItem>TaxRateArea = 10860</ComboBoxItem>
               <ComboBoxItem>Roll_LandValue &gt; 1000000</ComboBoxItem>
               <ComboBoxItem>Roll_LandValue &lt; 1000000</ComboBoxItem>
           </ComboBox>
   
       </Grid>
   

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3. In the section for the combo box you just added, right-click on the name of the event handler text ("QueryComboBox_SelectionChanged") and choose Go To definition from the context menu. Visual Studio will open the MapWindows.xaml.cs file with the stub for the new QueryComboBox_SelectionChanged() function.

Add code to call the selected query for the current map extent

When the user chooses an expression in the combo box, call a function to execute the query.

The function to execute the query takes three arguments: the ID for the layer to query (string), a SQL expression that defines attribute criteria (string), and the area of the MapView currently being viewed (Envelope).

1. In the QueryComboBox_SelectionChanged() function: modify the method signature to add the async keyword.

   MainWindow.xaml.cs

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           private async void QueryComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
           {
   
           }
   

2. Again, in the QueryComboBox_SelectionChanged() function: add code to gather the information required to perform the query, then call the QueryFeatureLayer function (that you will write in a coming step).

   MainWindow.xaml.cs

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           private async void QueryComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
           {
   
               // Since the first choice in the combo box are instructions for the app
               // (not actual SQL where clause syntax), ignore it.
               if (QueryComboBox.SelectedIndex != 0)
               {
                   // Get the view model using the ID given for the static resource in the XAML.
                   var currentMapViewModel = this.FindResource("MapViewModel") as MapViewModel;
   
                   // Get the current view point extent.
                   var currentExtent = MainMapView?.GetCurrentViewpoint(ViewpointType.BoundingGeometry)?.TargetGeometry as Envelope;
   
                   // Define the string ID for the feature layer to query.
                   string featureLayerId = "Parcels";
   
                   // Define the SQL query where clause to apply to the feature layer.
                   // Several SQL query strings were pre-populated in the combo box.
                   // This line of code gets the selected choice from the combo box
                   // and assigns it to the SQL query string.
                   object selectedValue = QueryComboBox.SelectedValue;
                   var sqlQuery = selectedValue?.ToString();
   
                   // Call the function in the view model that queries a feature layer. Pass in the:
                   // (1) feature layer ID
                   // (2) specific SQL syntax for the feature layer
                   // (3) current map extent
                   if (!string.IsNullOrEmpty(sqlQuery) && currentExtent != null)
                   {
                       await currentMapViewModel!.QueryFeatureLayer(featureLayerId, sqlQuery, currentExtent);
                   }
               }
   
           }
   

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Add the parcels layer to the map and set selection properties

You now need to begin working in a different code file within this project.

First, add the LA parcels FeatureLayer to the map's collection of data layers (GeoModel.OperationalLayers). Providing the Layer.Id allows for referencing the parcels layer from the layer collection when needed. Define the selection color via the map view's SelectionProperties to distinguish selected features in the map.

1. In the Visual Studio > Solution Explorer, double-click MapViewModel.cs to open the file.

   More info

   The project uses the Model-View-ViewModel (MVVM) design pattern to separate the application logic (view model) from the user interface (view). MapViewModel.cs contains the view model class for the application, called MapViewModel. See the Microsoft documentation for more information about the Model-View-ViewModel pattern.

2. Add additional required using statements at the top of the class. These make your code more concise and allow you to use classes from these namespaces without having to fully qualify them.

   MapViewModel.cs

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   using System;
   using System.Collections.Generic;
   using System.Text;
   using Esri.ArcGISRuntime.Geometry;
   using Esri.ArcGISRuntime.Mapping;
   using System.ComponentModel;
   using System.Runtime.CompilerServices;
   
   using Esri.ArcGISRuntime.Data;
   using Esri.ArcGISRuntime.UI;
   using System.Threading.Tasks;
   

3. Just after the section of code that defines the Map public property, create a similar SelectionProps public property as follows.

   MapViewModel.cs

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           private SelectionProperties? _selectionProps;
           public SelectionProperties? SelectionProps {
               get { return _selectionProps; }
               set
               {
                   _selectionProps = value;
                   OnPropertyChanged();
               }
           }
   

4. Add code to the SetupMap() function that adds the parcels feature layer to the map and sets the selection properties.

   MapViewModel.cs

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           private void SetupMap()
           {
               // Create a new map with a 'topographic vector' basemap.
               Map = new Map(BasemapStyle.ArcGISTopographic);
   
               // Add a layer that shows parcels for Los Angeles County.
               Uri parcelsUri = new Uri("https://services3.arcgis.com/GVgbJbqm8hXASVYi/arcgis/rest/services/LA_County_Parcels/FeatureServer/0");
               FeatureLayer parcelsFeatureLayer = new FeatureLayer(parcelsUri);
   
               // Give the layer an ID so we can easily find it later, then add it to the map.
               parcelsFeatureLayer.Id = "Parcels";
               Map.OperationalLayers.Add(parcelsFeatureLayer);
   
               // Create selection properties (bound to the MapView).
               SelectionProperties selectionProps = new SelectionProperties();
               selectionProps.Color = System.Drawing.Color.Yellow;
               this.SelectionProps = selectionProps;
   
           }
   

Create a function to query the parcels layer and select the result

In this step, create a new function that queries a FeatureLayer (identified using its ID) using both attribute and spatial criteria. After clearing any currently selected features, a new query will be executed to find features in the map's current extent that meet the selected attribute expression. The features in the FeatureQueryResult will be selected in the parcels layer.

1. Add the following new method named QueryFeatureLayer() just after the SetupMap() method.

   MapViewModel.cs

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           public async Task QueryFeatureLayer(string layerId, string whereExpression, Envelope queryExtent)
           {
               // Get the layer based on its Id.
               var featureLayerToQuery = _map?.OperationalLayers[layerId] as FeatureLayer;
   
               // Get the feature table from the feature layer.
               var featureTableToQuery = featureLayerToQuery?.FeatureTable;
               if(featureTableToQuery == null) { return; }
               // Clear any existing selection.
               featureLayerToQuery?.ClearSelection();
   
               // Create the query parameters using the where expression and extent passed in.
               QueryParameters queryParams = new QueryParameters
               {
                   Geometry = queryExtent,
                   ReturnGeometry = true,
                   WhereClause = whereExpression,
               };
   
               // Query the table and get the list of features in the result.
               var queryResult = await featureTableToQuery.QueryFeaturesAsync(queryParams);
   
               // Loop over each feature from the query result.
               foreach (Feature feature in queryResult)
               {
                   // Select each feature.
                   featureLayerToQuery!.SelectFeature(feature);
               }
           }
   

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2. Click Debug > Start Debugging (or press <F5> on the keyboard) to run the app.

The app loads with the map centered on the Santa Monica Mountains in California with the parcels feature layer displayed. Choose an attribute expression, and parcels in the current extent that meet the selected criteria will display in the specified selection color.

What's next?

Learn how to use additional API features, ArcGIS location services, and ArcGIS tools in these tutorials:

• Add a point, line, and polygon
• Add a feature layer