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 the Xcode project

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

2. Open the .xcodeproj file in Xcode.

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

   Instructions to set 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 Xcode, in the Project Navigator, click MainApp.swift.

   3. In the Editor, set the ArcGISEnvironment.apiKey property on the ArcGISEnvironment with your access token.

      MainApp.swift

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          init() {
              ArcGISEnvironment.apiKey = APIKey("<#YOUR-ACCESS-TOKEN#>")
          }
      

      Expand

Update the map

1. In Xcode, in the Project Navigator, click ContentView.swift.

2. In the editor, modify the map variable's initial viewpoint.

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           let map: Map = {
               let map = Map(basemapStyle: .arcGISTopographic)
   
               map.initialViewpoint = Viewpoint(latitude: 34.03000, longitude: -118.80500, scale: 6e4)
   
               return map
           }()
   

3. Create a private extension named Model with a class of type ObservableObject. Move the map variable there.

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   private extension ContentView {
   
       class Model: ObservableObject {
   
           let map: Map = {
               let map = Map(basemapStyle: .arcGISTopographic)
   
               map.initialViewpoint = Viewpoint(latitude: 34.03000, longitude: -118.80500, scale: 6e4)
   
               return map
           }()
   
   
       }
   
   }

4. Create a ServiceFeatureTable from the parcels service URL, then use it to make a FeatureLayer. Then, add the parcels feature layer to the map as an operational layer.

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       class Model: ObservableObject {
   
           let map: Map = {
               let map = Map(basemapStyle: .arcGISTopographic)
   
               map.initialViewpoint = Viewpoint(latitude: 34.03000, longitude: -118.80500, scale: 6e4)
   
               let parcelsURL = URL(string: "https://services3.arcgis.com/GVgbJbqm8hXASVYi/arcgis/rest/services/LA_County_Parcels/FeatureServer/0")!
               let featureTable = ServiceFeatureTable(url: parcelsURL)
               let featureLayer = FeatureLayer(featureTable: featureTable)
               map.addOperationalLayer(featureLayer)
   
               return map
           }()
   
   
       }
   

5. To access the model's map in the ContentView struct, create a private variable of type Model with a @StateObject property wrapper and modify the MapView to display the model's map.

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   struct ContentView: View {
   
       @StateObject private var model = Model()
   
       @State private var geoViewExtent: Envelope?
   
       var body: some View {
   
           MapView(map: model.map)
   
               .onVisibleAreaChanged { newVisibleArea in
                   geoViewExtent = newVisibleArea.extent
               }
   
       }
   
   }
   

Make an enumeration for predefined where clauses

To allow the user to query the parcels feature layer, create an enum that contains a set of predefined where clauses.

1. In the ContentView extension, make an enum type called WhereClause that is CaseIterable and Equatable. Add a switch statement and a case option for each where clause.

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   private extension ContentView {
   
       enum WhereClause: CaseIterable, Equatable {
           case government, residential, farm, taxRate10853, taxRate10860, landValueGreaterThan, landValueLessThan
   
           var expression: String {
               switch self {
               case .government: return "UseType = 'Government'"
               case .residential: return "UseType = 'Residential'"
               case .farm: return "UseType = 'Irrigated Farm'"
               case .taxRate10853: return "TaxRateArea = 10853"
               case .taxRate10860: return "TaxRateArea = 10860"
               case .landValueGreaterThan: return "Roll_LandValue > 1000000"
               case .landValueLessThan: return "Roll_LandValue < 1000000"
               }
           }
       }
   
       class Model: ObservableObject {
   
           let map: Map = {
               let map = Map(basemapStyle: .arcGISTopographic)
   
               map.initialViewpoint = Viewpoint(latitude: 34.03000, longitude: -118.80500, scale: 6e4)
   
               let parcelsURL = URL(string: "https://services3.arcgis.com/GVgbJbqm8hXASVYi/arcgis/rest/services/LA_County_Parcels/FeatureServer/0")!
               let featureTable = ServiceFeatureTable(url: parcelsURL)
               let featureLayer = FeatureLayer(featureTable: featureTable)
               map.addOperationalLayer(featureLayer)
   
               return map
           }()
   
   
       }
   
   }

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

Create a new function that queries the parcels FeatureTable with the chosen where clause and current extent. Clear any currently selected features and create a new QueryParameters object. Set the selectedWhereClause property to the option selected by the user and geoViewExtent to the current visible area. Asynchronously, query the feature table using the QueryParameters and select the features returned in the FeatureQueryResult.

1. Create a private variable of type Envelope with the @State property wrapper named geoViewExtent. This variable keeps track of the current, visible extent.

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   struct ContentView: View {
   
       @StateObject private var model = Model()
   
       @State private var geoViewExtent: Envelope?
   
       var body: some View {
   
           MapView(map: model.map)
   
       }
   
   }
   

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2. To update the geoViewExtent variable when the visible area is changed, add the onVisibleAreaChanged(perform:) task modifer to the map view. When the visible area has changed, update geoViewExtent to the new area's extent.

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   struct ContentView: View {
   
       @StateObject private var model = Model()
   
       @State private var geoViewExtent: Envelope?
   
       var body: some View {
   
           MapView(map: model.map)
   
               .onVisibleAreaChanged { newVisibleArea in
                   geoViewExtent = newVisibleArea.extent
               }
   
       }
   
   }
   

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3. Create a new method called queryFeatureLayer(selectedWhereClause:extent:) in the Model. Specify that it accepts parameters of type WhereClause and Envelope.

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       class Model: ObservableObject {
   
           let map: Map = {
               let map = Map(basemapStyle: .arcGISTopographic)
   
               map.initialViewpoint = Viewpoint(latitude: 34.03000, longitude: -118.80500, scale: 6e4)
   
               let parcelsURL = URL(string: "https://services3.arcgis.com/GVgbJbqm8hXASVYi/arcgis/rest/services/LA_County_Parcels/FeatureServer/0")!
               let featureTable = ServiceFeatureTable(url: parcelsURL)
               let featureLayer = FeatureLayer(featureTable: featureTable)
               map.addOperationalLayer(featureLayer)
   
               return map
           }()
   
   
           func queryFeatureLayer(with selectedWhereClause: WhereClause, in extent: Envelope) async {
               guard
                   let parcelsFeatureLayer = map.operationalLayers.first as? FeatureLayer,
                   let featureTable = parcelsFeatureLayer.featureTable
               else { return }
               parcelsFeatureLayer.clearSelection()
   
               let parameters = QueryParameters()
               parameters.whereClause = selectedWhereClause.expression
               parameters.geometry = extent
   
               do {
                   let result = try await featureTable.queryFeatures(using: parameters)
                   let features = result.features()
                   parcelsFeatureLayer.selectFeatures(features)
               } catch {
                   print(error)
               }
           }
   
       }
   

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Add UI for selecting a predefined where clause

To allow the user to choose a where clause that queries the parcels dataset, add a Picker that presents the list of predefined where clauses.

1. In the ContentView, create a @State variable named selectedWhereClause of type WhereClause. Assign .government as the default enum. The @State variable selectedWhereClause allows the model's queryFeatureLayer(selectedWhereClause:extent:) method to track which where clause the user has selected.

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   struct ContentView: View {
   
       @StateObject private var model = Model()
   
       @State private var geoViewExtent: Envelope?
   
       @State private var selectedWhereClause: WhereClause = .government
   
       var body: some View {
   
           MapView(map: model.map)
   
               .onVisibleAreaChanged { newVisibleArea in
                   geoViewExtent = newVisibleArea.extent
               }
   
       }
   
   }
   

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2. In the body, add a Toolbar and a ToolbarItemGroup with .bottomBar placement to the MapView.

   ContentView.swift

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       var body: some View {
   
           MapView(map: model.map)
   
               .onVisibleAreaChanged { newVisibleArea in
                   geoViewExtent = newVisibleArea.extent
               }
   
               .toolbar {
                   ToolbarItemGroup(placement: .bottomBar) {
   
                   }
               }
   
       }
   

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3. Add a Picker to the toolbar item group and label it, "Choose an SQL where clause". Set the selection to $selectedWhereClause. Iterating through .allCases of the WhereClause to populate the Picker with all the WhereClause enums.

   ContentView.swift

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       var body: some View {
   
           MapView(map: model.map)
   
               .onVisibleAreaChanged { newVisibleArea in
                   geoViewExtent = newVisibleArea.extent
               }
   
               .toolbar {
                   ToolbarItemGroup(placement: .bottomBar) {
   
                       Spacer()
                       Picker("Choose an SQL `where` clause", selection: $selectedWhereClause) {
                           ForEach(WhereClause.allCases, id: \.self) { clause in
                               Text(clause.expression)
                           }
   
                       }
                       Spacer()
   
                   }
               }
   
       }
   

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4. Lastly, add a .task modifier that calls the model's queryFeatureLayer(selectedWhereClause:extent:) function using the selectedWhereClause property.

   ContentView.swift

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       var body: some View {
   
           MapView(map: model.map)
   
               .onVisibleAreaChanged { newVisibleArea in
                   geoViewExtent = newVisibleArea.extent
               }
   
               .toolbar {
                   ToolbarItemGroup(placement: .bottomBar) {
   
                       Spacer()
                       Picker("Choose an SQL `where` clause", selection: $selectedWhereClause) {
                           ForEach(WhereClause.allCases, id: \.self) { clause in
                               Text(clause.expression)
                           }
   
                           .task(id: selectedWhereClause) {
                               guard let geoViewExtent = geoViewExtent else { return }
                               await model.queryFeatureLayer(with: selectedWhereClause, in: geoViewExtent)
                           }
   
                       }
                       Spacer()
   
                   }
               }
   
       }
   

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Run the app

Press Command + R 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 to display parcels that meet the selected criteria.

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