47
votes
Accepted
How to select features containing specific text string using an expression in QGIS
You just gotta use the LIKE operator.
For example,
"TITLE" LIKE '%MINERAL CLAIM%'
The % symbol acts like a wildcard.
LIKE is case-sensitive, whereas ILIKE is not.
- 9,921
35
votes
Accepted
What is the difference between != NULL and IS NOT NULL in QGIS filter expressions?
Disclaimer: Because the syntax for filtering in QGIS works with SQL, I'm assuming here that SQL rules apply. I'm not completely sure if that's entirely correct, but it seems logical and it does ...
- 2,271
24
votes
What is the difference between != NULL and IS NOT NULL in QGIS filter expressions?
NULL is not a value, therefore it cannot equal = or not equal != anything. It is not the same as zero 0 which is a value.
A NULL indicates that no value has been recorded in the cell you are looking ...
- 29.2k
20
votes
Accepted
Difference between quotation marks ('single' vs "double") in QGIS
Double quotes indicate that the string represents the name of an attribute while a single quote is a literal string.
So in your first case you get NULL because you don't have an attribute called ...
- 74.6k
17
votes
Accepted
Extracting part of string from field in QGIS Field Calculator?
Since the number of characters is same, you can use substr() function on a new field as in the following expression:
substr( "Location" ,17,6)
In the above example I used Path instead of ...
- 36.2k
17
votes
Using CASE statements to sort multiple values in QGIS Field Calculator
Use like this:
CASE
WHEN "stat_land_" IN ('A1', 'A2', 'A9', 'E', 'E2')
THEN 'Residential'
WHEN "stat_land_" IN ('C', 'C1')
THEN 'Vacant Land'
...
- 55.4k
16
votes
Accepted
Filling a field with a random string from a list in QGIS
You can build the array, build a random index and extract its value
array('a','b','c')[rand(0,2)]
- 34.3k
15
votes
Difference between quotation marks ('single' vs "double") in QGIS
Here is the answer directly from QGIS help:
column name "column name" → Value of the field column name, take care
to not be confused with simple quote, see below
'string' → a string value, ...
- 36.2k
15
votes
Accepted
Displaying only attributes not zero in QGIS Atlas text box
Manual version
With this option, you manually add a row for each attribute, reusing the same expression. See below how to automatize this for all fieldnames at once.
Create a condition for each ...
- 40k
14
votes
Accepted
QGIS query syntax differs between attribute table and feature filter
Your observation is correct, The Query Builder does not use the same syntax as the rest of QGIS.
Throughout QGIS, the syntax is based on QGIS Expressions a customized SQL dialect. This is portable ...
- 26.1k
14
votes
Accepted
Getting lat/long for centroids using QGIS
Inserting new fields with the field calculator and the expression $x and $y on the layer with your centroids should do the job. If you use the polygon layer of your lakes, you might include there lat/...
- 40k
13
votes
Accepted
Handling NULL values in labeling?
So the way QGIS handles NULL when you join strings is that it will convert the result to NULL if any part is NULL (yes I know it's on going debate on if it should or not)
Here is how you handle that ...
- 33.8k
13
votes
Accepted
QGIS - how to use geometry generator to place a centroid in each segment of a multiline?
You can use the following expression for Geometry Generator.
with_variable(
'segments', segments_to_lines($geometry),
collect_geometries(
array_foreach(
...
- 55.4k
13
votes
Accepted
Proper syntax of expression for selecting rows in QGIS
Try:
"binomial" = 'Axis axis'
In QGIS, double quotes " are used to reference a field. Single quotes ' are used to contain a string. No quotes for numerical values. The $ sign is used ...
- 24.6k
13
votes
Accepted
Using same color for label with same hour
Use Categorized symbology
Value: hour("time"), Color ramp: 'Random colors'
In Labels tab, use @symbol_color for the color data-defined override
- 3,469
12
votes
Accepted
Getting currentfeature's color in Expression Builder of QGIS?
In QGIS >= 2.14 you can use the symbol_color variable. Eg, set a data defined override for your label color to the expression @symbol_color
- 26.2k
12
votes
Adding text to end of existing value in QGIS
If your original field is a string-type field, you can update it instead of creating a new field.
Use the Field Calculator to update an existing field
Check the box next to "Update existing field"
...
- 23k
12
votes
Accepted
Place label along line near start and end of line in QGIS
2020 Oct update - label placement at start/end of lines or with a given offset is now be an inbuilt feature of QGIS 3.16 ! Link shows it in action.
QGIS 3.8 to 3.14
Consider using the geometry ...
- 7,304
12
votes
Filling a field with a random string from a list in QGIS
I'm not an expression expert. My solution uses a custom function.
Function definition:
from qgis.core import *
from qgis.gui import *
import random
@qgsfunction(args=-1, group='Custom')
def randstr(...
- 372
12
votes
Accepted
Getting attribute information of multiple polygon intersected by line
Try this expression:
array_to_string(overlay_intersects('Layer_A', name))
(assuming the City names are stored in name field in the 'Layer A').
- 29k
12
votes
Accepted
QGIS vector layer interactions: calculate percentage of polygon intersection area
Create a new attribute with Field calculator using this expression (replace orange on line 6 with the name of your "orange" layer):
area(
intersection(
$geometry,
...
- 40k
12
votes
Accepted
Sorting alphanumeric array numerically using QGIS Field Calculator
Create a new function using Function Editor.
Copy/paste the following script:
from qgis.core import *
from qgis.gui import *
import re
def natural_sort_key(s):
ns = re.compile('([0-9]+)')
...
- 55.4k
11
votes
Accepted
Is there an escape option for single quotes & apostrophes in QGIS?
Using the escaping by simply doubling the single quote works for me:
CASE
WHEN "NAME" = 'WILSON''S' THEN '0,75,224'
END
Double quotes in the field value don't need escaping:
CASE
WHEN "NAME" = '...
- 74.9k
11
votes
Accepted
Creating a label from two Attribute table column titles with a space between them
It should be
"First field" || ' ' || "Second field"
|| is string concatenation operator. Use Single quotes for Space, Double Quotes are for Field.
- 8,119
11
votes
Accepted
How to build a query/expression to select specific points within a buffer? (in QGIS)
This can be done using the Processing Graphical Modeler to set up a custom chain. Here's a pic:
From left to right we have three inputs - the tree layer (a Vector Layer Input Object), the buffer size ...
- 51.1k
11
votes
QGIS using project variables in filter queries
Layer filters don't use QGIS expressions, and are instead handed directly off to the underlying data provider. So, for instance, a filter on a postgis layer is passed to postgres to evaluate, and ...
- 26.2k
11
votes
Accepted
Labeling only one duplicate attribute without editing original data in QGIS
You can achieve something like in the picture just below by using some expressions without editing any of your data. Just use them as they are.
Follow the steps below and adjust them to your personal ...
- 24.6k
11
votes
Colouring points in the same colour as their nearest point/polygon from another layer in QGIS
You can use the new overlay expressions, available since QGIS version 3.16, see Visual changelog. By the way: I'm not sure what you mean that you have "a multi-polygon file with polygons and ...
- 40k
11
votes
Accepted
Adding new field in point layer with attribute from another layer in QGIS
You can use refFunctions Plugin. It adds custom user functions to QGIS Field Calculator. Then, you can add a new field to the point layer with the id of the polygon that contains it using the ...
- 55.4k
11
votes
Accepted
Use QGIS expressions to create geometries: difference between geometry generator and geometry by expression
Quick overview how to create new geometries using QGIS expressions with either Geometry generator or Geometry by expression:
Geometry generator: Open Layer Styling panel / add additional symbol layer (...
- 40k
Only top scored, non community-wiki answers of a minimum length are eligible