Neo4j 计数查询

Question

match(m:master_node:Application)-[r]-(k:master_node:Server)-[r1]-(n:master_node) 
where (m.name contains '' and (n:master_node:DeploymentUnit or n:master_node:Schema)) 
return distinct m.name,n.name

Hi,I am trying to get total number of records for the above query.How I change the query using count function to get the record count directly.

Thanks in advance

Answer 1

The following query uses the aggregating funtion COUNT . Distinct pairs of m.name, n.name values are used as the "grouping keys".

MATCH (m:master_node:Application)--(:master_node:Server)--(n:master_node) 
WHERE EXISTS(m.name) AND (n:DeploymentUnit OR n:Schema)
RETURN m.name, n.name, COUNT(*) AS cnt

I assume that m.name contains '' in your query was an attempt to test for the existence of m.name . This query uses the EXISTS() function to test that more efficiently.

[UPDATE]

To determine the number of distinct n and m pairs in the DB (instead of the number of times each pair appears in the DB):

MATCH (m:master_node:Application)--(:master_node:Server)--(n:master_node) 
WHERE EXISTS(m.name) AND (n:DeploymentUnit OR n:Schema)
WITH DISTINCT m.name AS n1, n.name AS n2
RETURN COUNT(*) AS cnt

Some things to consider for speeding up the query even further:

1. Remove unnecessary label tests from the MATCH pattern. For example, can we omit the master_node label test from any nodes? In fact, can we omit all label testing for any nodes without affecting the validity of the result? (You will likely need a label on at least one node, though, to avoid scanning all nodes when kicking off the query.)

2. Can you add a direction to each relationship (to avoid having to traverse relationships in both directions)?

3. Specify the relationship types in the MATCH pattern. This will filter out unwanted paths earlier. Once you do so, you may also be able to remove some node labels from the pattern as long as you can still get the same result.

4. Use the PROFILE clause to evaluate the number of DB hits needed by different Cypher queries.

Answer 2

You can find examples of how to use count in the Neo4j docs here

In your case the first example where:

count(*)

Is used to return a count of each returned item should work.