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Queries in QGIS IntroductionQGIS provides a number of different ways of selecting, filtering and querying map layers. It's important to understand the terms that QGIS uses to describe these different techniques - they are summarised in this table.Queries on layers can be used to filter the data to a smaller subset, select a subset of data for further processing, or simply to interrogate the data. QGIS uses specific terms for different actions: Action | Result | How | Details |
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Filter | Display only a subset of the features in a layer (including both map and attribute table), hiding hide the rest. | Layer > Filter | Write a query to filter features. Note that applying a filter to the attribute table filters the attribute table, but not the features on the map.Only the filtered data is used in QGIS | Select | Create a subset of tagged ( selected ) features from one or more layers, while continuing to display all features . Selected - selected features are highlighted in the map and attribute table. Further , and further actions (e.g. save, processing) can then be performed on the selection only. | | Click on one of the Select buttons, or use their dropdowns, to make a selection. | Identify | DShow Show data (attribute values) for a single feature or group of features, and optionally show values in a form for viewing/editing. Attribute data is shown for each feature, along with with derived data - e.g. length/area. | | Click the Identify features button, then click on a feature. |
FilterSelect the layer to be filteredFilter a layer to see only the filtered features - the user will not see any that do not any features which don’t meet the filter conditions , either on the map or in the attribute table. To apply a filter: will not be visible in QGIS. Apply a FilterOpen the essentials_03_styling project if it’s not open already Right-click > Filter on crime_bristol 202210 - neighbourhood crimes In the Query Builder dialog, use the Fields and Operators panels to construct a query
to show the features you needwhich will show only those neighbourhoods where total crime is over 200 Double-click on total_crime in the Fields list to bring it into the expression Click > Type 200
Review the result on the map and in the attribute table
For example, in the screenshot below, wind turbines have been filtered to those where the STATUS field has the value 'Application permitted - no conditions'.
The Query Builder dialog will help you construct a query interactively by selecting attributes, values and operators to build it up. It is similar but not identical to the Expression Based Filter dialog described below. To reset the query in order to see all data, click Clear in the Query Builder. SelectSelecting features 'tags' them for future use - once selected, a feature is highlighted in the map and in the attribute table. Selecting creates a subset of the data that QGIS can use to work with in other processes - for example, once features have been selected, you can: save the selected features as a new layer run processing tasks on theminvert the selection to select all features currently not selected. The Select tools are grouped together in the Attributes toolbar, and additional tools are in the Attribute Table toolbar, shown in the second row below. You can Select features: interactively in the map interface, for example by clicking or selecting by rectangle interactively in the attribute table, by clicking one or more rows by query, from the map or attribute table
To select interactively from Select featuresUse the selection tools to create some manual selections on the map,
click the dropdown (the black triangle) next to the Select features by area or single click button in the toolbar . The menu which appears gives you a number of options:
Select Feature(s) |
Select features individually | Select by rectangle |
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Click Select Feature(s) (it's selected by default), then click on the map to select a single feature at that location. Use Ctrl-click to select multiple features. | You can also click and drag a box over features to select all features that intersect the box. |
Select Features by Polygon/Freehand/Radius
Use these tools to select features by drawing a polygon, freehand line or radius:
Select by polygon | Select freehand | Select by radius |
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Select Features by Value or Expression
The tools on the Select Features by Value dropdown allow you to identify a selection by using a query, or by matching values against attributes.Select Features by Value displays a dialog where you can enter values to search for in the data - note that the search is interactive, so as soon as you start typing, matching values appear
Select by Expression displays a dialog in which you can build a query by selecting fields, operators and functions.
Navigate and Remove Selection
Navigate to a selection in two ways:
Pan Map to Selection will centre the map on the feature Zoom to Selection will set the map extents to match those of the feature To remove selections:
Deselect Features to deselect features from all layers The Attribute Table
The attribute table holds data about all the features in the layer - you can use it to view, edit and query data. Open it from the Attributes toolbar, or by right-click > Open Attribute table from a layer in the layer list. Each row in the attribute table corresponds to a feature on the map, and selection is synchronised between the table and the map - if a feature is selected in the attribute table, it's also selected on the map.
Attribute Table Views
The Show All Features button and compare the results in the attribute table for the layer - try the Select Features by Polygon and Select Features by Freehand options
In the wind_turbines_point layer, use the Select Features by Value dialog to select only those features where the YEAR was later than 2015
In the attribute table for the
Use the drop-down at the bottom left of the attribute table
is a button which has a dropdown with a number of options Show All Features: to
control which features appear in the table:Show
all features in the dataShow Selected Features
: Only features selected in the map/table will be shownShow Features Visible on Map: Only features which can be seen in the current map window are shown
Show Edited and New Features: During an editing session,
this shows features which have been created or edited but not yet savedField Filter: filter the table by selecting a field and a search value
Advanced Filter (Expression): filter the table by opening the Expression based filter dialog
Toggle Attribute Views
There are two possible views of the data within the attribute table - the default tabular or grid view, and the form view. Switch to the form view and back by using the buttons at the bottom right-hand side of the window.
The form view is similar to the form used in Identify Results, but includes a navigation panel on the left for moving between features. This can be more convenient for editing attribute values.
Form view
Select features in the form view of the attribute table by clicking the checkbox to the left of the feature list - once selected, the box for the feature will show yellow.
Attribute Table Field Filter
Run a simple search in the attribute table by selecting Field Filter from the menu in the bottom left of the attribute table dialog, then select the column name for the search. This is a simpler, single field version of the Select by Values tool described above, and it will show matching values interactively in the same way. To run the search, press Enter.
The attribute table will be filtered to show only the matching features.
Case sensitivity
Values are not case sensitive unless the Case sensitive box is checked, and the search is for a pattern within the string (for example ming will find Birmingham) so you don’t need the exact value.
Datatypes
Here, and in many other field-specific locations, there is an indicator of datatype immediately before the field name. For example, abc means a text field, while 123 means numeric.
Attribute Table Advanced Filter (Expression)
Select Advanced Filter (Expression) from the dropdown at bottom left of the attribute table to open the Expression Based Filter dialog - this is the same one as opened from Select Features by Expression earlier. However the result of applying this expression will only filter features in the attribute table, not on the map.
Queries can be built up in this dialog by selecting fields, operators, values and functions. For example, a simple query might be in the form:
[field] [operator] [value] - e.g. phase_of_education = 'Primary'
The example below builds a simple query to find all all parishes which are 'filler areas' - i.e. where there is no council.
In the Expression Based Filter dialog, navigate to Fields and Values in the central panel and select the relevant field (in this case descript0)
Double-click on the field to bring it into the Expression panel on the left
Click on the = operator in the Expression panel
In the Values panel, click all unique to show all the values of the field which appear in the data (or if there are a large number of values, click 10 samples)
Double-click on the value you want to use for the filter (in this case 'FILLER AREA')
You should now have an expression in place which looks something like the screenshot below - click OK to apply the filter
Operators
The operators below are the most commonly used, but more are available in the dialog.
Operator | Meaning |
|---|
= | Equal to |
<> | Not equal to |
< | Less than |
> | Greater than |
<= | Less than or equal to |
>= | Greater than or equal to |
LIKE | Case-sensitive string match |
ILIKE | Case-insensitive string match |
NOT | Combined with another operator to make it negative |
% | Wildcard – include in the search string at the point of uncertainty |
AND | Extends queries to apply more than one condition to a search |
OR | Extends queries to search for different values and will be returned if either is true |
|| | Join two values into a string |
The and check that your selection is correct - the status bar at the top of the table shows how many are selected
Right-click on the layer in the Layers panel and Export > Save Selected Features as… to save these stations as a new GeoPackage
In the original station_passenger_entries layer, clear the selection, and use the Select by Value option to select only those stations whose name begins with ‘London’, as per the screenshot below:
In the same dialog, use Zoom to Features to navigate to the selected stations
Identify
Use Identify to click on individual features on the map and get their attribute values, and derived data such as area and length.
Identify features
Click on crime_bristol_202210 — neighbourhood_crimes, then right-click > Zoom to Layer
Click on the Identify Features button to activate Identify, then click on one of the neighbourhoods - a panel will appear on the right with a list of values
Look at the options in the mini-toolbar at the top of the toolbar - note that you can copy the results to the clipboard
Open up the (Derived) section of the results to see the area and perimeter of the feature
Field Calculator
The Field Calculator creates new fields, or amends the values of existing ones , by using expressions . For - for example, you could create a new field which totals the values of two existing fields.Open
Create calculated fields
In the attribute table for crime_bristol_202210 — neighbourhood_crimes, open the Field Calculator by clicking the button in the
attribute table toolbar
. Choose whether to create Create a new field
or update an existing one, and use the expression builder to specify the contents of the target field - in the example above, the total of all crime types.As examples, you can use the Field Calculator to:
Rename a field: create a new field with the same type and length as the source field, with a new name, and populate the new field with data from the old field
Change data type: use a data type conversion (under Conversions) such as to_int to change a data type from string to numeric, for example
Derive coordinates: in the Geometry options, the $x and $y functions will return the coordinates of the points in a point layerstreet_crime, which is a total of asb, public_order, and theft_from_the_person
Use the settings in the screenshot below, and build up the formula by double-clicking on fields in the Fields and Values list
Click OK to add the new field, then check the attribute table to look at the new values
Spatial Selections and Filters
The queries and expressions we have been looking at until now work by using the attribute values of layers rather than their geography. There are a number of options for running spatially-based queries in QGIS, addressing Spatial queries answer questions such as:
Which points fall within this polygon?
Which lines cross this boundary?
What are the distances between features?
Two approaches are covered here:
Spatial queries
There are many types of spatial queries - use the search box at the left of the information bar to look for others.
Select by Location
This tool creates a new selection of features in a layer, based on a spatial relationship between the layer and another layer (or even the same layer). For example you might want to use it to:
select points in a polygon
select lines which intersect a polygon
select a polygon which contains a specific point.
To select points in a polygon:
Select the polygon you want to make the selection within - in this instance we have selected a parish within which we want to identify crimes
In the search box, search for Select by location, then double-click on it to bring up the dialog
In the Select by location dialog, complete the options in a similar way to the screenshot below
Note that:
The first layer should be the one which contains the things you want selected - e.g. crimes
In the checkboxes for the geometric predicate select the spatial relationship between the features in the two layers
The second layer is the one you are using to drive the selection - in this case parishes
The Selected features only checkbox specifies that only selected features in the second layer will be used to make the selection - in this case it's the single feature we selected in the first step
Click OK to run the selection - the result will be a map which looks something like this:
Note that only crimes within the selected parish have been selected, and are highlighted
Join Attributes by Location
This tool carries out a similar operation to the one above, but instead of making the result a selection, it adds the attribute values of the second layer to the first, target layer, so that each feature which satisfies the criteria in the spatial analysis takes on the attributes of the feature it is within (in this case). So for example, you could this to add a column to the crimes data, showing the parish that the crime took place in.
As in the previous example, select a polygon you want to make a selection with
In the search box, search for Join attributes by location
Open the dialog, and complete in a similar way to the screenshot below:
When the process is run a new Joined layer will be created and added to the map, comprising the original crime layer with, for each feature, the attributes from the parish it is within
This layer can then be retained as a temporary layer in the project, or saved as a new layer
Identify Features
The Identify Features tool is used to display the attribute values, and some derived properties, of a map feature.
Click the layer of interest in the layer list
Click the Identify Features button - the cursor will changeClick on a map feature.
The feature will be highlighted, and the Identify Results panel box will appear, showing the values of all the attributes for the feature
Derived Information
The Derived section in the panel includes measurements derived from the geometry of the feature such as length, area and perimeter (depending on the geometry type).
Multiple Features
If you click close to feature boundaries, multiple features are identified, and these will be listed as separate items in the Identify Results hierarchy. Browse the tree to see the data for each feature.
Feature Attributes
To view the attributes for a feature in a form dialog, click the View feature form button at the top of the Identify Results panel . This makes it easier to view the data, and if the layer is in edit mode, to edit it.To open the form automatically whenever Identify Results is used, check the Auto open form box at the bottom of the panel.
Identify Features from Multiple Layers
If you click on a location where there are features from multiple layers, you will be able to choose which layers to use in populating the Identify Results panel (including all of them).
To show results for a single selected layer, select Current layer from the Mode dropdown at the bottom of the Identify Results panel.
Copy to clipbboard
The Copy Selected Feature to clipboard button copies the data from the clicked feature to the clipboard as tab delimited text - you can then paste it to a text editor or directly into a spreadsheet.Create spatial queries
In the crime_bristol_202210 — neighbourhood_crimes layer, select those features where total_crime is greater than 100
Open the Select by Location dialog by searching in the search box (bottom left)
Select the railway stations which fall within the high crime neighbourhoods, using the settings in the screenshot below - note you will need to have Selected features only checked
Click Run, and look at the results in the railway_naptan layer - there should be four stations
Use Join Attributes by Location to add the attributes from the containing high crime neighbourhood to the station feature
Look at the attribute table for the stations to see the result (check Selected features only)
