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This commit adds a number of different forms of aggregates to
the expression engine.
1. Aggregates within the current layer, eg sum("passengers")
Supports sub expressions (ie sum("passengers"/2) ), group by
( sum("passengers", group_by:="line_segment") ), and optional
filters ( sum("passengers", filter:= "station_class" > 3 ) )
2. Relational aggregates, which calculate an aggregate over
all matching child features from a relation, eg
relation_aggregate( 'my_relation', 'mean', "some_child_field" )
3. A summary aggregate function, for calculating aggregates
on other layers. Eg aggregate('rail_station_layer','sum',"passengers")
The summary aggregate function supports an optional filter,
making it possible to calculate things like:
aggregate('rail_stations','sum',"passengers",
intersects(@atlas_geometry, $geometry ) )
for calculating the total number of passengers for the stations
inside the current atlas feature
In all cases the calculations are cached inside the expression
context, so they only need to be calculated once for each
set of expression evaluations.
Sponsored by Kanton of Zug, Switzerland
20 lines
2.2 KiB
Plaintext
20 lines
2.2 KiB
Plaintext
{
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"name": "aggregate",
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"type": "function",
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"description": "Returns an aggregate value calculated using features from another layer.",
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"arguments": [
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{"arg":"layer", "description":"a string, representing either a layer name or layer ID"},
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{"arg":"aggregate", "description":"a string corresponding to the aggregate to calculate. Valid options are:<br /><ul><li>count</li><li>count_distinct</li><li>count_missing</li><li>min</li><li>max</li><li>sum</li><li>mean</li><li>median</li><li>stdev</li><li>stdevsample</li><li>range</li><li>minority</li><li>majority</li><li>q1: first quartile</li><li>q3: third quartile</li><li>iqr: inter quartile range</li><li>min_length: minimum string length</li><li>max_length: maximum string length</li><li>concatenate: join strings with a concatenator</li></ul>"},
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{"arg":"calculation", "description":"sub expression or field name to aggregate"},
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{"arg":"filter", "optional":true, "description":"optional filter expression to limit the features used for calculating the aggregate"},
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{"arg":"concatenator", "optional":true, "description":"optional string to use to join values for 'concatenate' aggregate"}
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],
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"examples": [
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{ "expression":"aggregate(layer:='rail_stations',aggregate:='sum',expression:=\"passengers\")", "returns":"sum of all values from the passengers field in the rail_stations layer"},
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{ "expression":"aggregate('rail_stations','sum', \"passengers\"/7)", "returns":"calculates a daily average of \"passengers\" by dividing the \"passengers\" field by 7 before summing the values"},
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{ "expression":"aggregate(layer:='rail_stations',aggregate:='sum',expression:=\"passengers\")", "returns":"sum of all values from the passengers field in the rail_stations layer"},
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{ "expression":"aggregate(layer:='rail_stations',calculation:='sum',expression:=\"passengers\",filter:=\"class\">3)", "returns":"sums up all values from the \"passengers\" field from features where the \"class\" attribute is greater than 3 only"},
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{ "expression":"aggregate(layer:='rail_stations',calculation:='concatenate', expression:=\"name\", concatenator:=',')", "returns":"comma separated list of the name field for all features in the rail_stations layer"}
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]
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}
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