Questions tagged [virtual-layer]
A QGIS vector layer that is a database view built using SQL over other QGIS layers.
58
questions
6
votes
2answers
182 views
Using RECURSIVE in Virtual Layer
Preface
For a while, I was wondering about applying the looping/iterable
functions in QGIS's Virtual Layer. There were several cases when the
application of a loop would be beneficial and ...
0
votes
0answers
19 views
Using Virtual Raster File Viewer of QGIS?
I have created a virtual file using Build Virtual File functionality of QGIS.
Is the file linked strictly to where the raster files are stored or can it be viewed on another computer?
3
votes
2answers
74 views
Creating a virtual layer based on a virtual field for dynamic selection in QGIS 3.4?
Working on a PostGIS database, I would like to send to the print composer the selected features only. I'v tried several methods including working with is_selected() expression but the only way that ...
5
votes
1answer
212 views
Getting the line middle point with SQL in QGIS
Using SQL in a Virtual Layer is it possible to get the midpoint of a line on the line?
I have tried Finding middle point (midpoint) of line in QGIS? but got an error
I also tried
SELECT road, ...
3
votes
1answer
68 views
Using a virtual layer, display polygons based on location of another polygon layer
QGIS 3.4
I have a .gml file of every freehold title in a city outlined red below. I want to create a virtual layer of the freeholds which are contained within the blue-green isochrones. Even if a ...
1
vote
1answer
47 views
Changing SQL syntax in QGIS Virtual Layers
I am experiencing difficulties with SQL syntax in a Virtual Layer. I can do the simplest query form such as:
SELECT *
FROM mytable
and this displays on my canvas.
However, if I try to name fields ...
1
vote
2answers
42 views
Wrong result using virtual layer sum with join and group by
I am trying to join the grouped sum of two layers using virutal layers in QGIS 3.2.
My query looks like:
select nvewi.zone as "Zone-Type", sum(nvewi.bev) as "Inhabitants Isochrones", sum(nvewp.bev) ...
1
vote
1answer
52 views
Merging a legend and a table
I'm using QGIS 3.4 for this, and am creating a map that will see quite a lot of use. It will show plant species, and the area of the plants in question.
For this, we have a main "plant" table. This ...
2
votes
2answers
60 views
Selecting lines where start and end points within polygon using Virtual Layer
In QGIS 2.18 project there are several layers, lines and polygons respectively
My task is to select all lines when their start and end points are laying within polygons using Virtual Layer, see image ...
1
vote
0answers
49 views
Creating virtual layer or view table from original table using pivot table on QGIS?
I have a table and I need to produce a virtual layer with pivot or crosstable command.
Is it possible on QGIS 3.6 or 3.8?
My original sql is:
select cd_setor, Name, Path_relativo
from Fotos
order ...
2
votes
1answer
62 views
Merging shapefiles as Virtual Layer in QGIS
In QGIS 3.6.3 I'm trying to create a Virtual Layer obtained by merging 12 shapefiles (with the columns named and ordered in the same way) using the expression
SELECT *
FROM _January, _February, ...
3
votes
1answer
123 views
Difference between JOIN ON and WHERE in Virtual Layers in QGIS
There is already a question on this topic Difference between SpatialJoin with “…where…” and “join…on…”. However, it is dedicated to PostGIS.
Since in the Virtual Layer query window, I cannot simply ...
3
votes
2answers
280 views
Virtual layer functionality does not seem to support memory layers
I have a memory layer loaded in the QGIS virtual layer in the data source manager. I am trying to run a very simple query:
select * from layer
Here is a snapshot:
While this works for the physical ...
1
vote
2answers
52 views
write z-value from polygon layer to point layer in QGIS
I have two layers:
1 polygon layer where the depth of an area is written in the database (as DRVAL1) and and where the depth is exported as a Z-value to the geometry.
1 point layer that I use to plan ...
1
vote
1answer
103 views
Preserving geometry of Union output with Virtual Layer in QGIS 2
There is a polygon layer "Union23" that was created with 'Union' tool.
Layer's attribute table includes the following values.
My effort is simply to aggregate those polygons by "FLAECHEID" field ...
3
votes
1answer
74 views
Add virtual layer to aggregate feature
newbie here of QGIS here, I was trying to aggregate a feature from a CSV with no geometries. The query is very basic:
SELECT
"PROV",
SUM("POP_2012") AS POP_2012
FROM "municipalities-tuscany"
...
2
votes
1answer
82 views
UNION two LEFT JOINs as Virtual Layer in QGIS
In QGIS 2.18, there are two layers, e.g. "Locations" and "Connections"
"Locations" has values
Id | Dimension
---------------
1 | 4
2 | 8
3 | 2
"Connections" maintains attributes
...
4
votes
1answer
124 views
Select all columns except geometry using virtual layers
Is it possible to select all columns except geometry using virtual layers in QGIS?
For example I have this (simplified) query:
Select *, st_buffer(geometry, 100) From mypolygons
It does not work. ...
3
votes
0answers
66 views
QGIS Server Adding a virtual layer
All my layers in the qgs-project-file are working fine as WMS-layers, but I only can't provide the virtual layer.
The virtual layer is an ODBC-Connection of an Access-Database which contains one ...
1
vote
2answers
135 views
Creating a fixed distance buffer from a virtual layer
First off, i'm trying to learn some python to automate scripts for basic tasks i carry out in QGIS (2.18).
Below I create a virtual layer (polyline) using the following script:
from qgis.core ...
3
votes
1answer
132 views
Extending a segment in QGIS to check intersection with a polygon
I am trying to find the adress points (red dots) located in front of an obstacle (red polygon). The street is green and the pink line represents the street face (Si).
This is what I have so far, but ...
2
votes
2answers
172 views
Calculating point layer values within polygon features in QGIS 2
There are two layers
polygon BereichBerechnung with a field "Are_Number"
points EW2017 with a field "EWjeAdr"
In QGIS with a Virtual Layer, I want to calculate the sum of the field "EWjeAdr" for ...
1
vote
1answer
279 views
Efficient intersect query for virtual layers in QGIS
I have 3 tables/shapes as follows:
ADRR (adress points)
ADRR_ID | ST_ID (street surface id) | point geometry
STREET
ST_ID | ST_NAME (street name) | polygon geometry
Obstacles
ID_OBS | ...
2
votes
1answer
253 views
How to speed up a query in a virtual layer of QGIS?
I'm trying to speed up a query in a virtual layer.
I noticed that the query runs at the same time for a partial extension and full extent of the layer shown on the map. So I thought if I filtered the ...
1
vote
1answer
75 views
QGIS Custom Python Function to Return Current Extent QGIS 2.18LTR
Related Question Charts and Graphs that only display features in current extent
Trying to write a custom function to return the current view extent. The below code is what I wrote based on the QGIS ...
1
vote
0answers
25 views
Extract ordered point coordinates from polygon layer via Virtual Layer or PostGIS View
I have polygon layer and I need to create table with XY coordinates for each polygon staring from farthest to west (lowest value of X).
Now I have Virtual Layer with
SELECT ST_DissolvePoints(geometry)...
3
votes
1answer
65 views
How to load polygon layer with single field as virtual layer?
I have a polygon layer with 3 fields (A, B and C). I want to create a copy and add as a virtual layer so I use CREATE VIRTUAL LAYER option but only want the first field.
So I use the expression ...
2
votes
0answers
214 views
Unable to show geometry in virtual layer qgis
I'm trying to create a virtual layer starting from a municipality layer and from a table of attributes, both stored in a Postgresql DB.
So I've done a join like:
*SELECT munic, country_of_origin, ...
1
vote
0answers
109 views
Is it possible to add an atlas variable to the SQL query when creating a virtual layer?
I'm trying to do something like this:
select *
from table_a
inner join table_b
on table_b.fid = table_a.id
and table_b.code = @atlas_featureid
I have tried this using QGIS 2.18.16 and QGIS 3.0 ...
1
vote
0answers
367 views
Clipping Virtual Raster in QGIS 3 [closed]
I have generated a virtual raster out of a large number of tifs but I am only interested in an area so I want to clip it. When I do it, it saves a raster file of several Gb. I would like to clip the ...
1
vote
1answer
731 views
Spatial Query for a Virtual Layer
I would like to use SQL to create a virtual layer in QGIS by clipping information from a large data set using a bounding box around my study area.
I have two questions. Is this an efficient way to ...
5
votes
2answers
611 views
Keep virtual layer up to date
I have a virtual layer in QGIS with a query on a csv. The csv gets updated regulary. I need the max/min attributes in the virtual layer to be updated as well. The virtual layer is used as a title in ...
1
vote
1answer
201 views
QGIS: alternative to add/edit virtual layer. With python?
Following a previous question (Select polygons with other polygons layer?), I select some polygons using the query in the answer to create a virtual layer:
SELECT greens.* FROM greens, purples
WHERE ...
2
votes
1answer
166 views
Query and export a grouped subset of fields from a shapefile layer
I have several shapefile vector layers that I need to query so I can group records and export a subset of fields to go into an excel spreadsheet. I thought I could do this by using the graphical ...
2
votes
2answers
224 views
Getting nearest line to point with virtual layer using QGIS?
I have a streetmap and the position of hundred sensorlocations. I am currently trying to calculate the nearest line for each point. I have already done it with the nnjoin plugin. But now I want to do ...
1
vote
1answer
129 views
How to iterate nearest polygons until threshold is reached?
I have a point layer and a polygon layer (representing buildings). I created a buffer around the points and extracted those polygons which intersect the buffer.
The points contain an attribute with ...
3
votes
0answers
300 views
Virtual layer from query with special characters in field name using PyQGIS
I have a layer loaded from a WFS server that contains fields with æøå in some of the names. I am making SQL queries to make virtual layers from this layer, but whenever I query a field with æøå in its ...
2
votes
0answers
196 views
Virtual Layers Clipping and Large Data Sets in QGIS
When working with large vector data sets in QGIS is there a method where you can ask QGIS to only load a small subset of your data? I know that this can be done based on attributes using a virtual ...
3
votes
0answers
128 views
Identify Results form for virtual layers in QGIS keeps displaying wrong feature
When I load up a virtual layer and I use the Identify Results tool, it will display the correct information in the Identify Results panel, but in the feature form popup it will always display only the ...
7
votes
2answers
498 views
In QGIS is it possible to apply an expression to a join
I am trying to join by 2 text fields and there is a lot of mismatch due to CaSe and space issues... can it be run so that it's
lower("field1")=lower("field2")
I otherwise have to create a new field ...
2
votes
0answers
195 views
Using WFS layers in Virtual Layer crashing/too slow QGIS
I am trying to create a virtual layer with the following code, where the layers are provided by a WFS:
select geometry from (
select geometry from "Ancient Woodlands (England)" union all
select ...
5
votes
1answer
501 views
Can't export geometry generator styling to DXF - alternatives?
I've been using QGIS' wonderful geometry generator to dynamically generate root protection zones and crown areas for some tree inventories I've been working on. It's been a massive time saver.
...
1
vote
0answers
87 views
QGIS virtual layer: Linear referencing result with gaps
I have a layer with some roads, and a table, with information of whether the road is paved or unpaved.
The table contains linear referencing measurements from (vanpos) - to (totpos).
I'm using a ...
5
votes
0answers
97 views
Selecting minimum number of neighbouring polygons required to give a fixed total area
I have a polygon layer. I would like to select the minimum number of neighbouring polygons to give a total area greater than a fixed value. In reality the polygons are land parcels and I would like to ...
4
votes
1answer
642 views
Creating virtual layers using the processing toolbox
A few days ago Cy T posted an interesting question about creating virtual layers using the Python console:
Creating new Virtual Layer programmatically in QGIS?
Is it also possible to create virtual ...
1
vote
0answers
52 views
QGIS Virtual Layers based on WFS fail to load on re-opening project
I have created virtual layers that use WFS layers as one of their sources. Usually, it is an intersection between the WFS layer and a local polygon layer. I embed all the sources. However, when I ...
2
votes
1answer
435 views
How to save a virtual layer (from a filtered join) with features from original layer in QGIS?
To be specific, I have a shapefile on census block groups (CBGs) in Georgia, but I only want to use those in Atlanta's commuting zone (CZ).
However, commuting zone is not a field in the census ...
8
votes
2answers
2k views
Creating new Virtual Layer programmatically in QGIS?
Is it possible to create a Virtual Layer through a python script. For example, I have a layer "road", and I would like to perform sql " SELECT * FROM road WHERE type = 'Expressway' "
Will this be ...
1
vote
1answer
161 views
How to get QgsFeatureRequest in a virtual Layer?
I am wondering if/how PyQGIS can intercept the QgsVirtualLayerProvider::getFeatures() requests of a VirtualLayer, and pull out just the spatial extent from the QgsFeatureRequest to dynamically ...
3
votes
1answer
261 views
How to include layer names when union'ing in virtual layers?
I am creating a virtual layer in QGIS using the following statement:
SELECT * FROM (
SELECT * FROM a UNION ALL
SELECT * FROM b UNION ALL
SELECT * FROM c
) AS merged_tables
What is the best way to ...
