How to use QGIS in a virtual layer query?

How to use QGIS in a virtual layer query?

Functions of QGIS expressions can also be used in a virtual layer query. To refer the geometry column of a layer, use the name geometry. Contrary to a pure SQL query, all the fields of a virtual layer query must be named. Don’t forget to use the as keyword to name your columns if they are the result of a computation or function call.

Which is the list of functions in QGIS?

Returns the count of missing (NULL) values. Returns the calculated inter quartile range from a field or expression. Returns the aggregate majority of values (most commonly occurring value) from a field or expression. group_by – optional expression to use to group aggregate calculations

When to use the keyword geometry in QGIS?

To refer the geometry column of a layer, use the name geometry. Contrary to a pure SQL query, all the fields of a virtual layer query must be named. Don’t forget to use the as keyword to name your columns if they are the result of a computation or function call.

What can be used in a virtual layer query?

The underlying engine uses SQLite and Spatialite to operate. It means you can use all of the SQL your local installation of SQLite understands. Functions from SQLite and spatial functions from Spatialite can also be used in a virtual layer query.

How to create a new GeoPackage layer in QGIS?

QGIS will automatically add the new layer to the legend, and you can edit it in the same way as described in section Digitizing an existing layer. Further management of SpatiaLite layers can be done with the DB Manager. See DB Manager Plugin. To create a new GeoPackage layer go to Layer ‣ New ‣ New GeoPackage Layer… .

What do you call a virtual layer in SQL?

These layers are called virtual layers: they do not carry data by themselves and can be seen as views to other layers. Open the virtual layer creation dialog by clicking on Add Virtual Layer in the Layer menu or from the corresponding toolbar. The dialog allows you to specify an SQL query.

What are the changes to the QGIS API?

If you’re interested, you can read about some of the rationale behind the API changes on this mailing list thread: [Qgis-developer] new vector api select features. Briefly (in the words of QGIS developer Martin Dobias):

Why is there no f.attributemap ( ) in QGIS?

Briefly (in the words of QGIS developer Martin Dobias): Access to attributes: there is no f.attributeMap () anymore, because attributes are now stored in a vector (Python: list) instead of a map (Python: dict ).

What is the use of indexes in QGIS?

Use of indexes A special kind of vector layer allows you to define a layer as the result of an advanced query, using the SQL language on any number of other vector layers that QGIS is able to open. These layers are called virtual layers: they do not carry data by themselves and can be seen as views to other layers.

How to create a virtual layer in SQL?

For example, if you have a layer called regions, you can create a new virtual layer with an SQL query like SELECT * FROM regions WHERE id < 100 . The SQL query will be executed, whatever the underlying provider of the regions layer is and even if this provider does not directly support SQL queries.

What happens to temporary scratch layers in QGIS?

Temporary Scratch Layers are in-memory layers, meaning that they are not saved on disk and will be discarded when QGIS is closed. They can be handy to store features you temporarily need or as intermediate layers during geoprocessing operations.

Which is an example of an expression in QGIS?

Based on layer data and prebuilt or user defined functions, Expressions offer a powerful way to manipulate attribute value, geometry and variables in order to dynamically change the geometry style, the content or position of the label, the value for diagram, the height of a composer item, select some features, create virtual field

Why are there no data values in QGIS?

This layer contains values of accumulated flow, a hydrological parameter. It contains those values only within the area of a given watershed, with no–data values outside of it. As you can see, the rendering is not very informative, due to the way values are distributed.

How to create a new scratch layer in QGIS?

To create a new Temporary Scratch layer, choose the New Temporary Scratch Layer… entry in the Layer ► Create Layer ► menu or in the Data Source Manager toolbar. The New Temporary Scratch Layer dialog will be displayed as shown in Fig. 13.18.

How to run a QGIS processing algorithm in Python?

You can also run a QGIS Processing algorithm via Python using the processing.runAndLoadResults () function instead of processing.run () as shown above – which will load the result to QGIS canvas directly.

How does the ftools plugin work in QGIS?

fTools is now automatically installed and enabled in new versions of QGIS, and as with all plugins, it can be disabled and enabled using the Plugin Manager (see The Plugins Dialog ). When enabled, the fTools plugin adds a Vector menu to QGIS, providing functions ranging from Analysis and Research Tools to Geometry…

What is topology check in geometry plugin QGIS?

Topology checks: checks for duplicates, for features within other features, overlaps smaller than a number, for gaps smaller than a number. Tolerance: you can define here the tolerance for the check. Output vector layer gives the choice to the user how get the result between modifiy the current layer and create a new layer.

How does the query builder work in QGIS?

The Query Builder allows you to define a subset of a table using a SQL-like WHERE clause and to display the result in the main window. The query result can then be saved as a new vector layer. Open the Query Builder by opening the Layer Properties and going to the General menu.

How to use geomnearest and dbvalue in QGIS?

Here are the layers and attribute tables. refFunctions plugin to use geomnearest () and dbvalue (). QGIS version: 2.16 or 2.18 to use concatenate () expression in aggregate group. As your Lines-Points is in 1-N relationship, most Field Calculator tasks have to be done on Point layer.

How to list the values of an attribute in QGIS?

To list all possible values of an attribute, select the attribute in the Fields list and click the [all] button. To list the first 25 unique values of an attribute column, select the attribute column in the Fields list and click the [Sample] button. To add a value to the SQL WHERE clause field, double click its name in the Values list.

How to create a qlineedit widget in Python?

Here is a minimal example (can be loaded into QGIS as a script) how to create QLineEdit widget, connnect it correctly with function and show changing text while editing the entry: Not the answer you’re looking for? Browse other questions tagged qgis python pyqt unicode or ask your own question.

How to draw country polygons in QGIS 4.5?

Here we create an object of the QgsGraduatedSymbolRenderer class that we want to use to draw the country polygons from our layer using a graduated color approach based on the population attribute ‘POP2005’. The name of the field to use is set via the renderer’s setClassAttribute () method in line 2.