Expressions
The Expressions feature are available through the field calculator or the add a new column button in the attribut table or the Field tab in the Layer properties ; through the graduaded, categorized and rule-based rendering in the Style tab of the Layer properties ; through the expression-based labeling 
in the 
Labeling core application ; through the feature selection and through the diagram tab of the Layer properties as well as the Main properties of the label item and the Atlas generation tab in the Print Composer.
They are a powerful way to manipulate attribute value in order to dynamically change the final value in order to change the geometry style, the content of the label, the value for diagram, select some feature or create virtual column.
Functions List
The Function List contains functions as well as fields and values. View the help function in the Selected Function Help. In Expression you see the calculation expressions you create with the Function List. For the most commonly used operators, see Operators.
In the Function List, click on Fields and Values to view all attributes of the attribute table to be searched. To add an attribute to the Field calculator Expression field, double click its name in the Fields and Values list. Generally, you can use the various fields, values and functions to construct the calculation expression, or you can just type it into the box. To display the values of a field, you just right click on the appropriate field. You can choose between Load top 10 unique values and Load all unique values. On the right side, the Field Values list opens with the unique values. To add a value to the Field calculator Expression box, double click its name in the Field Values list.
The Operators, Math, Conversions, String, Geometry and Record groups provide several functions. In Operators, you find mathematical operators. Look in Math for mathematical functions. The Conversions group contains functions that convert one data type to another. The String group provides functions for data strings. In the Geometry group, you find functions for geometry objects. With Record group functions, you can add a numeration to your data set. To add a function to the Field calculator Expression box, click on the > and then double click the function.
Operators
This group contains operators (e.g., +, -, *).
a + b a plus b
a - b a minus b
a * b a multiplied by b
a / b a divided by b
a % b a modulo b (for example, 7 % 2 = 1, or 2 fits into 7 three
times with remainder 1)
a ^ b a power b (for example, 2^2=4 or 2^3=8)
a = b a and b are equal
a > b a is larger than b
a < b a is smaller than b
a <> b a and b are not equal
a != b a and b are not equal
a <= b a is less than or equal to b
a >= b a is larger than or equal to b
a ~ b a matches the regular expression b
+ a positive sign
- a negative value of a
|| joins two values together into a string 'Hello' || ' world'
LIKE returns 1 if the string matches the supplied pattern
ILIKE returns 1 if the string matches case-insensitive the supplied
pattern (ILIKE can be used instead of LIKE to make the match
case-insensitive)
IS returns 1 if a is the same as b
OR returns 1 when condition a or b is true
AND returns 1 when condition a and b are true
NOT returns 1 if a is not the same as b
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, take care to not be
confused with double quote, see above
NULL null value
a IS NULL a has no value
a IS NOT NULL a has a value
a IN (value[,value]) a is below the values listed
a NOT IN (value[,value]) a is not below the values listed
Some examples:
-
Joins a string and a value from a column name:
'My feature's id is: ' || "gid" -
Test if the “description” attribute field starts with the ‘Hello’ string in the value (note the position of the % character):
"description" LIKE 'Hello%'
Conditionals
This group contains functions to handle conditional checks in expressions.
CASE evaluates multiple expressions and returns a
result
CASE ELSE evaluates multiple expressions and returns a
result
coalesce returns the first non-NULL value from the
expression list
regexp_match returns true if any part of a string matches
the supplied regular expression
Some example:
- Send back a value if the first condition is true, else another value:
CASE WHEN "software" LIKE '%QGIS%' THEN 'QGIS' ELSE 'Other'
Mathematical Functions
This group contains math functions (e.g., square root, sin and cos).
sqrt(a) square root of a
abs returns the absolute value of a number
sin(a) sine of a
cos(a) cosine of a
tan(a) tangent of a
asin(a) arcsin of a
acos(a) arccos of a
atan(a) arctan of a
atan2(y,x) arctan of y/x using the signs of the two
arguments to determine the quadrant of the
result
exp exponential of a value
ln value of the natural logarithm of the passed
expression
log10 value of the base 10 logarithm of the passed
expression
log value of the logarithm of the passed value
and base
round round to number of decimal places
rand random integer within the range specified by
the minimum
and maximum argument (inclusive)
randf random float within the range specified by
the minimum
and maximum argument (inclusive)
max largest value in a set of values
min smallest value in a set of values
clamp restricts an input value to a specified
range
scale_linear transforms a given value from an input
domain to an output
range using linear interpolation
scale_exp transforms a given value from an input
domain to an output
range using an exponential curve
floor rounds a number downwards
ceil rounds a number upwards
$pi pi as value for calculations
Conversions
This group contains functions to convert one data type to another (e.g., string to integer, integer to string).
toint converts a string to integer number
toreal converts a string to real number
tostring converts number to string
todatetime converts a string into Qt data time type
todate converts a string into Qt data type
totime converts a string into Qt time type
tointerval converts a string to an interval type (can be
used to take days, hours, months, etc. off a
date)
Date and Time Functions
This group contains functions for handling date and time data.
$now current date and time
age difference between two dates
year extract the year part from a date, or the number of years from
an interval
month extract the month part from a date, or the number of months
from an interval
week extract the week number from a date, or the number of weeks
from an interval
day extract the day from a date, or the number of days from an
interval
hour extract the hour from a datetime or time, or the number
of hours from an interval
minute extract the minute from a datetime or time, or the number
of minutes from an interval
second extract the second from a datetime or time, or the number
of minutes from an interval
Some example:
- Get the month and the year of today in the format “10/2014”
month($now) || '/' || year($now)
String Functions
This group contains functions that operate on strings (e.g., that replace, convert to upper case).
lower convert string a to lower case
upper convert string a to upper case
title converts all words of a string to title
case (all words lower case with leading
capital letter)
trim removes all leading and trailing white
space (spaces, tabs, etc.) from a string
wordwrap returns a string wrapped to a maximum/
minimum number of characters
length length of string a
replace returns a string with the supplied string
replaced
regexp_replace(a,this,that) returns a string with the supplied regular
expression replaced
regexp_substr returns the portion of a string which matches
a supplied regular expression
substr(*a*,from,len) returns a part of a string
concat concatenates several strings to one
strpos returns the index of a regular expression
in a string
left returns a substring that contains the n
leftmost characters of the string
right returns a substring that contains the n
rightmost characters of the string
rpad returns a string with supplied width padded
using the fill character
lpad returns a string with supplied width padded
using the fill character
format formats a string using supplied arguments
format_number returns a number formatted with the locale
separator for thousands (also truncates the
number to the number of supplied places)
format_date formats a date type or string into a custom
string format
Color Functions
This group contains functions for manipulating colors.
color_rgb returns a string representation of a color based on its
red, green, and blue components
color_rgba returns a string representation of a color based on its
red, green, blue, and alpha (transparency) components
ramp_color returns a string representing a color from a color ramp
color_hsl returns a string representation of a color based on its
hue, saturation, and lightness attributes
color_hsla returns a string representation of a color based on its
hue, saturation, lightness and alpha (transparency)
attributes
color_hsv returns a string representation of a color based on its
hue, saturation, and value attributes
color_hsva returns a string representation of a color based on its
hue, saturation, value and alpha (transparency) attributes
color_cmyk returns a string representation of a color based on its
cyan, magenta, yellow and black components
color_cmyka returns a string representation of a color based on its
cyan, magenta, yellow, black and alpha (transparency)
components
Geometry Functions
This group contains functions that operate on geometry objects (e.g., length, area).
$geometry returns the geometry of the current feature (can be used
for processing with other functions)
$area returns the area size of the current feature
$length returns the length size of the current feature
$perimeter returns the perimeter length of the current feature
$x returns the x coordinate of the current feature
$y returns the y coordinate of the current feature
xat retrieves the nth x coordinate of the current feature.
n given as a parameter of the function
yat retrieves the nth y coordinate of the current feature.
n given as a parameter of the function
xmin returns the minimum x coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
xmax returns the maximum x coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
ymin returns the minimum y coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
ymax returns the maximum y coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
geomFromWKT returns a geometry created from a well-known text (WKT)
representation
geomFromGML returns a geometry from a GML representation of geometry
bbox
disjoint returns 1 if the geometries do not share any space
together
intersects returns 1 if the geometries spatially intersect
(share any portion of space) and 0 if they don't
touches returns 1 if the geometries have at least one point in
common, but their interiors do not intersect
crosses returns 1 if the supplied geometries have some, but not
all, interior points in common
contains returns true if and only if no points of b lie in the
exterior of a, and at least one point of the interior
of b lies in the interior of a
overlaps returns 1 if the geometries share space, are of the
same dimension, but are not completely contained by
each other
within returns 1 if geometry a is completely inside geometry b
buffer returns a geometry that represents all points whose
distance from this geometry is less than or equal to
distance
centroid returns the geometric center of a geometry
bounds returns a geometry which represents the bounding box of
an input geometry. Calculations are in the Spatial
Reference System of this Geometry.
bounds_width returns the width of the bounding box of a geometry.
Calculations are in the Spatial Reference System of
this Geometry.
bounds_height returns the height of the bounding box of a geometry.
Calculations are in the Spatial Reference System of
this Geometry.
convexHull returns the convex hull of a geometry (this represents
the minimum convex geometry that encloses all geometries
within the set)
difference returns a geometry that represents that part of geometry
a that does not intersect with geometry b
distance returns the minimum distance (based on spatial ref)
between two geometries in projected units
intersection returns a geometry that represents the shared portion
of geometry a and geometry b
symDifference returns a geometry that represents the portions of a and
b that do not intersect
combine returns the combination of geometry a and geometry b
union returns a geometry that represents the point set union of
the geometries
geomToWKT returns the well-known text (WKT) representation of the
geometry without SRID metadata
geometry returns the feature's geometry
transform returns the geometry transformed from the source CRS to
the dest CRS
Record Functions
This group contains functions that operate on record identifiers.
$rownum returns the number of the current row
$id returns the feature id of the current row
$currentfeature returns the current feature being evaluated.
This can be used with the 'attribute' function
to evaluate attribute values from the current
feature.
$scale returns the current scale of the map canvas
$uuid generates a Universally Unique Identifier (UUID)
for each row. Each UUID is 38 characters long.
getFeature returns the first feature of a layer matching a
given attribute value.
attribute returns the value of a specified attribute from
a feature.
$map returns the id of the current map item if the map
is being drawn in a composition, or "canvas" if
the map is being drawn within the main QGIS
window.
Fields and Values
Contains a list of fields from the layer. Sample values can also be accessed via right-click.
Select the field name from the list, then right-click to access a context menu with options to load sample values from the selected field.
Fields name should be double-quoted. Values or string should be simple-quoted.