To calculate aggregated data, Cypher offers aggregation, much like SQL’s GROUP BY.
If any aggregation functions are found in the RETURN statement, all the columns without aggregating functions are used as the grouping key.
Graph
COUNT is used to count the number of rows. COUNT can be used in two forms - COUNT(*)
which just counts the number of matching rows, and COUNT(<identifier>), which counts
the number of non-null values in <identifier>.
To count the number of nodes, for example the number of nodes connected to one node, you can use count(*).
Query
START n=node(2) MATCH (n)-->(x) RETURN n, count(*)
The start node and the count of related nodes.
To count the groups of relationship types, return the types and count them with count(*).
Query
START n=node(2) MATCH (n)-[r]->() RETURN type(r), count(*)
The relationship types and their group count.
Instead of counting the number of results with count(*), it might be more expressive to include the name of the identifier you care about.
Query
START n=node(2) MATCH (n)-->(x) RETURN count(x)
The number of connected nodes from the start node.
You can count the non-null values by using count(<identifier>).
Query
START n=node(2,3,4,1) RETURN count(n.property?)
The count of related nodes.
The SUM aggregation function simply sums all the numeric values it encounters. Null values are silently dropped. This is an example of how you can use SUM.
Query
START n=node(2,3,4) RETURN sum(n.property)
The sum of all the values in the property property.
AVG calculates the average of a numeric column.
Query
START n=node(2,3,4) RETURN avg(n.property)
The average of all the values in the property property.
MAX find the largets value in a numeric column.
Query
START n=node(2,3,4) RETURN max(n.property)
The largest of all the values in the property property.
MIN takes a numeric property as input, and returns the smallest value in that column.
Query
START n=node(2,3,4) RETURN min(n.property)
The smallest of all the values in the property property.
COLLECT collects all the values into a list.
Query
START n=node(2,3,4) RETURN collect(n.property)
Returns a single row, with all the values collected.
All aggregation functions also take DISTINCT modifier, which removes duplicates from the values.
So, to count the number of unique eye colours from nodes related to a, this query can be used:
Query
START a=node(2) MATCH a-->b RETURN count(distinct b.eyes)
Returns the number of eye colours.