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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
17 lines
1.6 KiB
Plaintext
17 lines
1.6 KiB
Plaintext
{
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"name": "relation_aggregate",
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"type": "function",
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"description": "Returns an aggregate value calculated using all matching child features from a layer relation.",
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"arguments": [
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{"arg":"relation", "description":"a string, representing a relation 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":"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":"relation_aggregate(relation:='my_relation',aggregate:='mean',expression:=\"passengers\")", "returns":"mean value of all matching child features using the 'my_relation' relation"},
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{ "expression":"relation_aggregate('my_relation','sum', \"passengers\"/7)", "returns":"sum of the passengers field divided by 7 for all matching child features using the 'my_relation' relation"},
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{ "expression":"relation_aggregate('my_relation','concatenate', \"towns\", concatenator:=',')", "returns":"comma separated list of the towns field for all matching child features using the 'my_relation' relation"}
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]
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}
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