Learn how to execute a SQL query to access polygon features from a feature layer.
A feature layer can contain a large number of features stored in ArcGIS. To access a subset of the features, you can execute a SQL or spatial query, or both at the same time. You can also return the attributes, geometry, or both attributes and geometry for each record. SQL and spatial queries are useful when a feature layer is very large and you just want to access a subset of the data.
In this tutorial, you perform server-side SQL queries to return a subset of the features from the LA County Parcel hosted feature layer. The feature layer contains over 2.4 million features. The resulting features are displayed as graphics on the map. A pop-up is also used to display feature attributes.
To access location services, you need an API key or OAuth 2.0 access token. To learn how to create and scope your key, visit the Create an API key tutorial.
Go to your dashboard to get an API key. The API key must be scoped to access the services used in this tutorial.
In CodePen, update apiKey to use your key.
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const apiKey = "YOUR_API_KEY";
const basemapEnum = "arcgis/streets";
const map = new maplibregl.Map({
container: "map", // the id of the div elementstyle: `https://basemapstyles-api.arcgis.com/arcgis/rest/services/styles/v2/styles/${basemapEnum}?token=${apiKey}`,
zoom: 12, // starting zoomcenter: [-118.805, 34.027] // starting location [longitude, latitude]});
Add API references
This tutorial uses ArcGIS REST JS to query the feature layer.
Hosted feature layers support a standard SQL query where clause. Use an HTML <select> element to provide a list of SQL queries for the LA County Parcel feature layer.
To add the <select> HTML element as a Map control, you create an object that implements the MapLibre GL JS IControl interface.
Creating an IControl allows MapLibre GL JS to position it, which prevents your control overlapping other MapLibre GL JS controls.
Create a QueryControl class with an onAdd function. Inside, create a <div> element with a <select> inside, with options for each of the SQL where clauses. Return this element.
Parcels returned by the query are simple GeoJSON polygons. You can display them with a GeoJSON source and a fill layer.
Create an addParcelLayer function. Inside, add a GeoJSON source and a fill layer. Add the layer before the first symbol layer.
By inserting the polygon layer beneath the icon and text layers, it keeps the labels readable. In MapLibre GL JS, there is no distinction between "basemap" layers and "overlay layers", so you can insert the layer anywhere.
To determine the first symbol layer, you can use map.getStyle to get the style, then iterate to find the first layer whose type is symbol.
Use the ArcGIS REST JS queryFeatures method to find features in the LA County Parcels feature layer that match the selected where clause.
To limit the query results to the visible extent of the Map, you can set the queryFeaturesgeometry property. To do so, use map.getBounds to get the extent of the Map viewport, and then convert it to the required esriGeometryEnvelope format of [minx, miny, maxx, maxy]. This confines the SQL query to the geographic bounds (extent) of the map.
When the matching parcels are returned, call setData on the parcels source to display them.
Create a function called executeQuery with whereClause and geometry parameters. Inside, create a new arcgisRest.ApiKeyManager to access the feature service. Call arcgisRest.queryFeatures. Pass the geometry and geometryType. Specify GeoJSON as the return type, requesting returnGeometry and specific outFields. All of the features within the geometry will be returned with attribute information set by the outFields property.
There are many other spatial relationships that you can specify with spatialRel. For example, you can use esriSpatialRelContains to only return parcels completely within the viewport. See the ArcGIS services reference for details.
Update your QueryControl to add a change event handler to the select element. Inside, calculate the viewport extent and call executeQuery with the extent and where clause.
At the top right, click Run. When you select a where clause using the toolbox, a query will run against the feature layer and display all land parcels within the current viewport matching the where clause.
Add a pop-up
You can add a Popup to view attributes of a parcel when the user clicks on it.
Add a click event handler to the parcels-fill layer. Inside, construct the pop-up content from the attributes of the clicked parcel. Create a new Popup, then use popup.setHTML to set the content. Set the position of the pop-up and add it to the map.
When the map displays, you should be able to use the query selector to display parcels. Click on a parcel to show a pop-up with the feature's attributes.