Querying a graph with Cypher

Cypher, Neo4j's language: patterns, relationship direction, variable-length paths, aggregates, CREATE and MERGE.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Pattern: what is it?
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An ASCII description of a shape to find in the graph: nodes in parentheses linked by relationships in square brackets and arrows.
(a:Personne)-[:AMI]->(b) reads “a person a and a node b such that a has an AMI relationship to b”; the arrow's direction matters.
The graph holds directed AMI relationships: Léa→Tom, Léa→Inès, Léa→Paul, Tom→Zoé, Inès→Zoé, Zoé→Léa. What does MATCH (p)-[:AMI]->(:Personne {nom:'Léa'}) RETURN count(p) count?
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The arrow ends at Léa: it counts those who name her as a friend. Only Zoé does. The three friends Léa names would be counted by reversing the arrow.
In Cypher, the … clause filters a pattern's occurrences, and the … clause chooses the result's columns.
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In Cypher, the WHERE clause filters a pattern's occurrences, and the RETURN clause chooses the result's columns.
MATCH (p:Personne) WHERE p.age > 30 RETURN p.nom chains the three clauses in that order.
MATCH … RETURN: what is it?
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The two clauses of a read: the first finds every occurrence of a pattern, the second chooses what is returned for each.
Each occurrence of the pattern gives a row: a node reached by two paths appears twice, unless RETURN DISTINCT.