memgraph/docs/user_technical/open-cypher.md

openCypher Query Language

openCypher is a query language for querying graph databases. It aims to be intuitive and easy to learn, while providing a powerful interface for working with graph based data.

Memgraph supports most of the commonly used constructs of the language. This chapter contains the details of features which are implemented. Additionally, not yet supported features of the language are listed.

• Reading Existing Data
• Writing New Data
• Reading & Writing
• Indexing
• Other Features

Reading Existing Data

The simplest usage of the language is to find data stored in the database. For that purpose, the following clauses are offered:

• MATCH, which searches for patterns;
• WHERE, for filtering the matched data and
• RETURN, for defining what will be presented to the user in the result set.

MATCH

This clause is used to obtain data from Memgraph by matching it to a given pattern. For example, to find each node in the database, you can use the following query.

MATCH (node) RETURN node

Finding connected nodes can be achieved by using the query:

MATCH (node1) -[connection]- (node2) RETURN node1, connection, node2

In addition to general pattern matching, you can narrow the search down by specifying node labels and properties. Similarly, edge types and properties can also be specified. For example, finding each node labeled as Person and with property age being 42, is done with the following query.

MATCH (n :Person {age: 42}) RETURN n.

While their friends can be found with the following.

MATCH (n :Person {age: 42}) -[:FriendOf]- (friend) RETURN friend.

There are cases when a user needs to find data which is connected by traversing a path of connections, but the user doesn't know how many connections need to be traversed. openCypher allows for designating patterns with variable path lengths. Matching such a path is achieved by using the * (asterisk) symbol inside the pattern for a connection. For example, traversing from node1 to node2 by following any number of connections in a single direction can be achieved with:

MATCH (node1) -[*]-> (node2)

If paths are very long, finding them could take a long time. To prevent that, a user can provide the minimum and maximum length of the path. For example, paths of length between 2 and 4 can be obtained with a query like:

MATCH (node1) -[*2..4]-> (node2)

More details on how MATCH works can be found here. Note that named paths are not yet supported.

The MATCH clause can be modified by prepending the OPTIONAL keyword. OPTIONAL MATCH clause behaves the same as a regular MATCH, but when it fails to find the pattern, missing parts of the pattern will be filled with null values. Examples can be found here.

WHERE

You have already seen that simple filtering can be achieved by using labels and properties in MATCH patterns. When more complex filtering is desired, you can use WHERE paired with MATCH or OPTIONAL MATCH. For example, finding each person older than 20 is done with the this query.

MATCH (n :Person) WHERE n.age > 20 RETURN n

Additional examples can be found here.

RETURN

The RETURN clause defines which data should be included in the resulting set. Basic usage was already shown in the examples for MATCH and WHERE clauses. Another feature of RETURN is renaming the results using the AS keyword.

Example.

MATCH (n :Person) RETURN n AS people

That query would display all nodes under the header named people instead of n.

When you want to get everything that was matched, you can use the * (asterisk) symbol.

This query:

MATCH (node1) -[connection]- (node2) RETURN *

is equivalent to:

MATCH (node1) -[connection]- (node2) RETURN node1, connection, node2

RETURN can be followed by the DISTINCT operator, which will remove duplicate results. For example, getting unique names of people can be achieved with:

MATCH (n :Person) RETURN DISTINCT n.name

Besides choosing what will be the result and how it will be named, the RETURN clause can also be used to:

• limit results with LIMIT sub-clause;
• skip results with SKIP sub-clause;
• order results with ORDER BY sub-clause and
• perform aggregations (such as count).

More details on RETURN can be found here.

SKIP & LIMIT

These sub-clauses take a number of how many results to skip or limit. For example, to get the first 3 results you can use this query.

MATCH (n :Person) RETURN n LIMIT 3

If you want to get all the results after the first 3, you can use the following.

MATCH (n :Person) RETURN n SKIP 3

The SKIP and LIMIT can be combined. So for example, to get the 2nd result, you can do:

MATCH (n :Person) RETURN n SKIP 1 LIMIT 1

ORDER BY

Since the patterns which are matched can come in any order, it is very useful to be able to enforce some ordering among the results. In such cases, you can use the ORDER BY sub-clause.

For example, the following query will get all :Person nodes and order them by their names.

MATCH (n :Person) RETURN n ORDER BY n.name

By default, ordering will be in the ascending order. To change the order to be descending, you should append DESC.

For example, to order people by their name descending, you can use this query.

MATCH (n :Person) RETURN n ORDER BY n.name DESC

You can also order by multiple variables. The results will be sorted by the first variable listed. If the values are equal, the results are sorted by the second variable, and so on.

Example. Ordering by first name descending and last name ascending.

MATCH (n :Person) RETURN n ORDER BY n.name DESC, n.lastName

Note that ORDER BY sees only the variable names as carried over by RETURN. This means that the following will result in an error.

MATCH (n :Person) RETURN old AS new ORDER BY old.name

Instead, the new variable must be used:

MATCH (n: Person) RETURN old AS new ORDER BY new.name

The ORDER BY sub-clause may come in handy with SKIP and/or LIMIT sub-clauses. For example, to get the oldest person you can use the following.

MATCH (n :Person) RETURN n ORDER BY n.age DESC LIMIT 1

Aggregating

openCypher has functions for aggregating data. Memgraph currently supports the following aggregating functions.

• avg, for calculating the average.
• collect, for collecting multiple values into a single list or map. If given a single expression values are collected into a list. If given two expressions, values are collected into a map where the first expression denotes map keys (must be string values) and the second expression denotes map values.
• count, for counting the resulting values.
• max, for calculating the maximum result.
• min, for calculating the minimum result.
• sum, for getting the sum of numeric results.

Example, calculating the average age:

MATCH (n :Person) RETURN avg(n.age) AS averageAge

Collecting items into a list:

MATCH (n :Person) RETURN collect(n.name) AS list_of_names

Collecting items into a map:

MATCH (n :Person) RETURN collect(n.name, n.age) AS map_name_to_age

Click here for additional details on how aggregations work.

Writing New Data

For adding new data, you can use the following clauses.

• CREATE, for creating new nodes and edges.
• SET, for adding new or updating existing labels and properties.
• DELETE, for deleting nodes and edges.
• REMOVE, for removing labels and properties.

You can still use the RETURN clause to produce results after writing, but it is not mandatory.

Details on which kind of data can be stored in Memgraph can be found in Storable Data Types chapter.

CREATE

This clause is used to add new nodes and edges to the database. The creation is done by providing a pattern, similarly to MATCH clause.

For example, to create 2 new nodes connected with a new edge, use this query.

CREATE (node1) -[:edge_type]-> (node2)

Labels and properties can be set during creation using the same syntax as in MATCH patterns. For example, creating a node with a label and a property:

CREATE (node :Label {property: "my property value"}

Additional information on CREATE is here.

SET

The SET clause is used to update labels and properties of already existing data.

Example. Incrementing everyone's age by 1.

MATCH (n :Person) SET n.age = n.age + 1

Click here for a more detailed explanation on what can be done with SET.

DELETE

This clause is used to delete nodes and edges from the database.

Example. Removing all edges of a single type.

MATCH () -[edge :type]- () DELETE edge

When testing the database, you want to often have a clean start by deleting every node and edge in the database. It is reasonable that deleting each node should delete all edges coming into or out of that node.

MATCH (node) DELETE node

But, openCypher prevents accidental deletion of edges. Therefore, the above query will report an error. Instead, you need to use the DETACH keyword, which will remove edges from a node you are deleting. The following should work and delete everything in the database.

MATCH (node) DETACH DELETE node

More examples are here.

REMOVE

The REMOVE clause is used to remove labels and properties from nodes and edges.

Example.

MATCH (n :WrongLabel) REMOVE n :WrongLabel, n.property

Reading & Writing

OpenCypher supports combining multiple reads and writes using the WITH clause. In addition to combining, the MERGE clause is provided which may create patterns if they do not exist.

WITH

The write part of the query cannot be simply followed by another read part. In order to combine them, WITH clause must be used. The names this clause establishes are transferred from one part to another.

For example, creating a node and finding all nodes with the same property.

CREATE (node {property: 42}) WITH node.property AS propValue
MATCH (n {property: propValue}) RETURN n

Note that the node is not visible after WITH, since only node.property was carried over.

This clause behaves very much like RETURN, so you should refer to features of RETURN.

MERGE

The MERGE clause is used to ensure that a pattern you are looking for exists in the database. This means that if the pattern is not found, it will be created. In a way, this clause is like a combination of MATCH and CREATE.

Example. Ensure that a person has at least one friend.

MATCH (n :Person) MERGE (n) -[:FriendOf]-> (m)

The clause also provides additional features for updating the values depending on whether the pattern was created or matched. This is achieved with ON CREATE and ON MATCH sub clauses.

Example. Set a different properties depending on what MERGE did.

MATCH (n :Person) MERGE (n) -[:FriendOf]-> (m)
ON CREATE SET m.prop = "created" ON MATCH SET m.prop = "existed"

For more details, click this link.

Indexing

An index stores additional information on certain types of data, so that retrieving said data becomes more efficient. Downsides of indexing are:

• requiring extra storage for each index and
• slowing down writes to the database.

Carefully choosing which data to index can tremendously improve data retrieval efficiency, and thus make index downsides negligible.

Memgraph automatically indexes labeled data. This improves queries which fetch nodes by label:

MATCH (n :Label) ... RETURN n

Indexing can also be applied to data with a specific combination of label and property. These are not automatically created, instead a user needs to create them explicitly. Creation is done using a special CREATE INDEX ON :Label(property) language construct.

For example, to index nodes which is labeled as :Person and has a property named age:

CREATE INDEX ON :Person(age)

After the index is created, retrieving those nodes will become more efficient. For example, the following query will retrieve all nodes which have an age property, instead of fetching each :Person node and checking whether the property exists.

MATCH (n :Person {age: 42}) RETURN n

Using index based retrieval also works when filtering labels and properties with WHERE. For example, the same effect as in the previous example can be done with:

MATCH (n) WHERE n:Person AND n.age = 42 RETURN n

Since the filter inside WHERE can contain any kind of an expression, the expression can be complicated enough so that the index does not get used. We are continuously improving the recognition of index usage opportunities from a WHERE expression. If there is any suspicion that an index may not be used, we recommend putting properties and labels inside the MATCH pattern.

Currently, once an index is created it cannot be deleted. This feature will be implemented very soon. The expected syntax for removing an index will be DROP INDEX ON :Label(property).

Note that Memgraph does not support concurrent index building. Attempting to build an index from two concurrent transactions is likely to result in an error (reported to the client) for one of those transactions, regardless of index label and property. Do not create indices concurrently.

Other Features

The following sections describe some of the other supported features.

UNWIND

The UNWIND clause is used to unwind a list of values as individual rows.

Example. Produce rows out of a single list.

UNWIND [1,2,3] AS listElement RETURN listElement

More examples are here.

Functions

You have already been introduced to one type of functions, aggregating functions. This section contains the list of other supported functions.

Name	Description
coalesce	Returns the first non null argument.
startNode	Returns the starting node of an edge.
endNode	Returns the destination node of an edge.
degree	Returns the number of edges (both incoming and outgoing) of a node.
head	Returns the first element of a list.
last	Returns the last element of a list.
properties	Returns the properties of a node or an edge.
size	Returns the number of elements in a list.
toBoolean	Converts the argument to a boolean.
toFloat	Converts the argument to a floating point number.
toInteger	Converts the argument to an integer.
type	Returns the type of an edge as a character string.
keys	Returns a list keys of properties from an edge or a node. Each key is represented as a string of characters.
labels	Return a list of labels from a node. Each label is represented as a character string.
range	Constructs a list of value in given range.
tail	Returns all elements after the first of a given list.
abs	Returns the absolute value of a number.
ceil	Returns the smallest integer greater than or equal to given number.
floor	Returns the largest integer smaller than or equal to given number.
round	Returns the number, rounded to the nearest integer. Tie-breaking is done using the commercial rounding, where -1.5 produces -2 and 1.5 produces 2.
exp	Calculates e^n where e is the base of the natural logarithm, and n is the given number.
log	Calculates the natural logarithm of a given number.
log10	Calculates the logarithm (base 10) of a given number.
sqrt	Calculates the square root of a given number.
acos	Calculates the arccosine of a given number.
asin	Calculates the arcsine of a given number.
atan	Calculates the arctangent of a given number.
atan2	Calculates the arctangent2 of a given number.
cos	Calculates the cosine of a given number.
sin	Calculates the sine of a given number.
tan	Calculates the tangent of a given number.
sign	Applies the signum function to a given number and returns the result. The signum of positive numbers is 1, of negative -1 and for 0 returns 0.
e	Returns the base of the natural logarithm.
pi	Returns the constant pi.
rand	Returns a random floating point number between 0 (inclusive) and 1 (exclusive).
startsWith	Check if the first argument starts with the second.
endsWith	Check if the first argument ends with the second.
contains	Check if the first argument has an element which is equal to the second argument.
all	Check if all elements of a list satisfy a predicate. The syntax is: all(variable IN list WHERE predicate).
assert	Raises an exception reported to the client if the given argument is not true.

String Operators

Apart from comparison and concatenation operators openCypher provides special string operators for easier matching of substrings:

Operator	Description
a STARTS WITH b	Returns true if prefix of string a is equal to string b.
a ENDS WITH b	Returns true if suffix of string a is equal to string b.
a CONTAINS b	Returns true if some substring of string a is equal to string b.

Parameters

When automating the queries for Memgraph, it comes in handy to change only some parts of the query. Usually, these parts are values which are used for filtering results or similar, while the rest of the query remains the same.

Parameters allow reusing the same query, but with different parameter values. The syntax uses the $ symbol to designate a parameter name. We don't allow old Cypher parameter syntax using curly brace. For example, you can parameterize filtering a node property:

MATCH (node1 {property: $propertyValue}) RETURN node1

You can use parameters instead of any literal in the query, but not instead of property maps even though that is allowed in standard openCypher. Following example is illegal in Memgraph:

MATCH (node1 $propertyValue) RETURN node1

To use parameters with Python driver use following syntax:

session.run('CREATE (alice:Person {name: $name, age: $ageValue}',
            name='Alice', ageValue=22)).consume()

To use parameters which names are integers you will need to wrap parameters in a dictionary and convert them to strings before running a query:

session.run('CREATE (alice:Person {name: $0, age: $1}',
            {'0': "Alice", '1': 22})).consume()

To use parameters with some other driver please consult appropriate documentation.

CASE

Conditional expressions can be expressed in openCypher language by simple and generic form of CASE expression. A simple form is used to compare an expression against multiple predicates. For the first matched predicate result of the expression provided after the THEN keyword is returned. If no expression is matched value following ELSE is returned is provided, or null if ELSE is not used:

MATCH (n)
RETURN CASE n.currency WHEN "DOLLAR" THEN "$" WHEN "EURO" THEN "€" ELSE "UNKNOWN" END

In generic form, you don't provided expression whose value is compared to predicates, but you list multiple predicates and the first one that evaluates to true is matched:

MATCH (n)
RETURN CASE WHEN n.height < 30 THEN "short" WHEN n.height > 300 THEN "tall" END

Differences

Although we try to implement openCypher query language as closely to the language reference as possible, we had to make some changes to enhance the user experience.

Symbolic Names

We don't allow symbolic names (variables, label names...) to be openCypher keywords (WHERE, MATCH, COUNT, SUM...).

Matching the Same Edge

We support using the same edge name to match that edge across the whole pattern. For example:

MATCH ()-[r]-()-[r]-() RETURN r

The above would find the edge r which forms a circular connection on a node. This behaviour is not supported in openCypher reference and the query would fail.

Unicode codepoints in string literal

Use \u followed by 4 hex digits in string literal for UTF-16 codepoint and '\U' with 8 hex digits for UTF-32 codepoint in memgraph.