Query a table to get aggregated statistics back for a specific field.
Use case
For example, a county boundaries table with population information can be queried to return aggregated results for total, average, maximum, and minimum population, rather than downloading the values for every county and calculating statistics manually.
How to use the sample
Pan and zoom to define the extent for the query. Use the 'Cities in current extent' checkbox to control whether the query only includes features in the visible extent. Use the 'Cities grater than 5M' checkbox to filter the results to only those cities with a population greater than 5 million people. Click 'Get statistics' to perform the query. The query will return population-based statistics from the combined results of all features matching the query criteria.
How it works
- Create a
ServiceFeatureTable
with a URL to the feature service. - Create
StatisticsQueryParameters
, andStatisticDefinition
objects, and add to the parameters. - Execute
queryStatistics
on theServiceFeatureTable
. Depending on the state of the two checkboxes, additional parameters are set. - Display each
StatisticRecord
in the first returnedQueryStatisticsResult
.
Relevant API
- FeatureLayer
- QueryParameters
- ServiceFeatureTable
- StatisticDefinition
- StatisticRecord
- StatisticsQueryParameters
- StatisticsQueryResult
- StatisticType
Tags
analysis, average, bounding geometry, filter, intersect, maximum, mean, minimum, query, spatial query, standard deviation, statistics, sum, variance
Sample Code
// [WriteFile Name=StatisticalQuery, Category=Analysis]
// [Legal]
// Copyright 2017 Esri.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// http://www.apache.org/licenses/LICENSE-2.0
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// [Legal]
import QtQuick 2.6
import QtQuick.Controls 2.2
import QtQuick.Layouts 1.3
import Esri.ArcGISRuntime 100.15
Rectangle {
clip: true
width: 800
height: 600
readonly property url worldCitiesUrl: "https://sampleserver6.arcgisonline.com/arcgis/rest/services/SampleWorldCities/MapServer/0"
readonly property alias extentOnly: extentCheckbox.checked
readonly property alias bigCitiesOnly: bigCitiesCheckbox.checked
property string resultsText: ""
MapView {
id: mapView
anchors.fill: parent
Component.onCompleted: {
// Set the focus on MapView to initially enable keyboard navigation
forceActiveFocus();
}
// Create a new Map with the world streets vector basemap
Map {
Basemap {
initStyle: Enums.BasemapStyleArcGISStreets
}
// Create a new feature layer to display features in the world cities table
FeatureLayer {
// Create feature table using the world cities URL
ServiceFeatureTable {
id: featureTable
url: worldCitiesUrl
// Setup signal handler for queryStatisticsStatusChanged signal
onQueryStatisticsStatusChanged: {
if (queryStatisticsStatus !== Enums.TaskStatusCompleted)
return;
if (!queryStatisticsResult)
return;
// Reset the result text
resultsText = "";
// Iterate through the results
const iter = queryStatisticsResult.iterator;
while (iter.hasNext) {
const record = iter.next();
for (let key in record.statistics) {
if (record.statistics.hasOwnProperty(key)) {
resultsText += key + ": " + record.statistics[key] + "\n";
}
}
}
// Display the results
msgDialog.open();
}
onErrorChanged: {
resultsText = error.message;
msgDialog.open();
}
}
}
}
}
Rectangle {
anchors {
fill: controlColumn
margins: -10
}
color: "#E9E9EA"
radius: 3
opacity: 0.8
border {
color: "black"
width: 1
}
}
Column {
id: controlColumn
anchors {
left: parent.left
top: parent.top
margins: 15
}
spacing: 5
CheckBox {
id: extentCheckbox
text: "Only cities in current extent"
}
CheckBox {
id: bigCitiesCheckbox
text: "Only cities greater than 5M"
}
Button {
anchors.horizontalCenter: parent.horizontalCenter
text: "Get Statistics"
onClicked: {
// If only using features in the current extent, set up the spatial filter for the statistics query parameters
if (extentOnly) {
// Set the statistics query parameters geometry with the envelope
queryParameters.geometry = mapView.currentViewpointExtent.extent;
// Set the spatial relationship to Intersects (which is the default)
queryParameters.spatialRelationship = Enums.SpatialRelationshipIntersects;
} else {
// Reset the query parameters
queryParameters.geometry = null;
}
// If only evaluating the largest cities (over 5 million in population), set up an attribute filter
if (bigCitiesOnly)
queryParameters.whereClause = "POP_RANK = 1";
else
queryParameters.whereClause = "1=1";
// Execute the statistical query with these parameters
featureTable.queryStatistics(queryParameters);
}
}
}
// Declare a StatisticsQueryParameters object
StatisticsQueryParameters {
id: queryParameters
// Add the Statistic Definitions
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeAverage}
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeMinimum}
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeMaximum}
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeSum}
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeStandardDeviation}
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeVariance}
StatisticDefinition { onFieldName: "POP"; statisticType: Enums.StatisticTypeCount; outputAlias: "CityCount"}
}
// Create a dialog to display the result
Dialog {
id: msgDialog
modal: true
x: Math.round(parent.width - width) / 2
y: Math.round(parent.height - height) / 2
standardButtons: Dialog.Ok
property alias text : textLabel.text
property alias informativeText : detailsLabel.text
ColumnLayout {
Text {
id: textLabel
text: "Query Statistics Results:"
}
Text {
id: detailsLabel
text: resultsText
}
}
}
}