Spanner Graph queries overview

This document describes how to query property graphs in Spanner Graph. The examples in this section use the graph schema that you create in Set up and query Spanner Graph, which is illustrated in the following diagram:

Run a Spanner Graph query

You can run Spanner Graph queries in the following ways:

• The Google Cloud console

  Submit a query on the Spanner Studio page. To access the Spanner Studio page, click Spanner Studio from the Database overview page or Table overview page. For more information about accessing Spanner Studio, see Manage your data using the Google Cloud console.

• The gcloud spanner command-line tool

  Submit a command by using the gcloud spanner databases execute-sql command.

• The Spanner client libraries

• The executeSql and executeStreamingSql REST API

• The ExecuteSql and ExecuteStreamingSql RPC API

Spanner Graph query structure

This section describes each query component in detail.

The following example illustrates the basic structure of a Spanner Graph query.

Spanner Graph lets you create multiple graphs inside a database. The query starts by specifying the target graph, FinGraph, using the GRAPH clause.

Graph pattern matching

Graph pattern matching finds specific patterns within your graph. The most basic patterns are Element patterns (node patterns and edge patterns), which match graph elements (nodes and edges, respectively). Element patterns can be composed into path patterns and more complex patterns.

Node patterns

A node pattern is a pattern that matches nodes from your graph. This pattern comprises a matching pair of parentheses, which might optionally contain a graph pattern variable, a label expression, and property filters.

Find all nodes

The following query returns all nodes in the graph. The variable n, called a graph pattern variable, binds to the matching nodes. In this case, the node pattern matches all nodes in the graph.

GRAPH FinGraph
MATCH (n)
RETURN LABELS(n) AS label, n.id;

Result

The query returns label and id as follows:

Find all nodes with a specific label

The following query matches all nodes in the graph that have the Person label. The query returns the label and id, name properties of the matched nodes.

GRAPH FinGraph
MATCH (p:Person)
RETURN LABELS(p) AS label, p.id, p.name;

Result

Find all nodes matching a label expression

You can create a label expression with one or more logical operators.

The following query matches all nodes in the graph that have either the Person or Account label. The set of properties exposed by the graph pattern variable n is the superset of the properties exposed by the nodes that have either the Person or Account label.

GRAPH FinGraph
MATCH (n:Person|Account)
RETURN LABELS(n) AS label, n.id, n.birthday, n.create_time;

• In the results, all nodes have the id property.
• Nodes matching Account label have the create_time property, but don't have the birthday property. A NULL is returned for the birthday property for such nodes.
• Nodes matching Person label have the birthday property, but don't have the create_time property. A NULL is returned for the create_time property for such nodes.

Result

For more information on label expression rules, see Label expression.

Find all nodes matching the label expression and property filter

The following query matches all nodes in the graph that have the Person label, and where the property id is equal to 1.

GRAPH FinGraph
MATCH (p:Person {id: 1})
RETURN LABELS(p) AS label, p.id, p.name, p.birthday;

Result

You can use the WHERE clause to form more complex filtering conditions on labels and properties.

The following query matches all nodes in the graph that have the Person label, and the property birthday is before 1990-01-10.

GRAPH FinGraph
MATCH (p:Person WHERE p.birthday < '1990-01-10')
RETURN LABELS(p) AS label, p.name, p.birthday;

Result

Edge patterns

An edge pattern matches edges or relationships between nodes. Edge patterns are enclosed with square brackets [] with symbols -, ->, or <- to indicate directions.

Similar to node patterns, graph pattern variables are used to bind to matching edge elements.

Find all edges with matching labels

The following query returns all edges in the graph that have the Owns label. The graph pattern variable e is bound to the matching edges.

GRAPH FinGraph
MATCH -[e:Owns]->
RETURN e.id AS owner_id, e.account_id;

Result

Find all edges matching the label expression and property filter

Similar to a node pattern, an edge pattern can use label expressions, property specification, and WHERE clauses, as shown in the following query. The query finds all edges labeled with Owns and has the property create_time in a specified period.

GRAPH FinGraph
MATCH -[e:Owns WHERE e.create_time > '2020-01-14'
                 AND e.create_time < '2020-05-14']->
RETURN e.id AS owner_id, e.create_time, e.account_id;

Result

Find all edges using any direction edge pattern

Although all edges in Spanner Graph are directed, you can use the any direction edge pattern -[]- in a query to match edges in either direction.

The following query finds all transfers where a blocked account is involved.

GRAPH FinGraph
MATCH (account:Account)-[transfer:Transfers]-(:Account)
WHERE account.is_blocked
RETURN transfer.order_number, transfer.amount;

Result

Path patterns

A path pattern is built from alternating node and edge patterns.

Find all paths from a node with specified label and property filters, using a path pattern

The following query finds all transfers to an account initiated from an account owned by Person with id equal to 2.

Each matched result represents a path from Person {id: 2} through a connected Account using the Owns edge, into another Account using the Transfers edge.

GRAPH FinGraph
MATCH
  (p:Person {id: 2})-[:Owns]->(account:Account)-[t:Transfers]->
  (to_account:Account)
RETURN
  p.id AS sender_id, account.id AS from_id, to_account.id AS to_id;

Result

Quantified path patterns

A quantified pattern allows a pattern to be repeated within a specified range.

Match a quantified edge pattern

The following query finds all the destination accounts one to three transfers away from a source Account with id equal to 7, other than itself.

The edge pattern postfixed with the quantifier {1, 3}.

GRAPH FinGraph
MATCH (src:Account {id: 7})-[e:Transfers]->{1, 3}(dst:Account)
WHERE src != dst
RETURN src.id AS src_account_id, ARRAY_LENGTH(e) AS path_length, dst.id AS dst_account_id;

Result

The previous example uses the ARRAY_LENGTH function to access the group variable e. For more information, see access group variable.

Some rows in the example results are repeated because there might be multiple paths between the same pair of src and dst accounts that match the pattern.

Match a quantified path pattern

The following query finds paths between Account nodes with one to two Transfers edges through intermediate accounts that are blocked.

The parenthesized path pattern is quantified and the WHERE clause is used in the parenthesis to specify conditions for the repeated pattern.

GRAPH FinGraph
MATCH
  (src:Account)
  ((:Account)-[:Transfers]->(interm:Account) WHERE interm.is_blocked){1,2}
    -[:Transfers]->(dst:Account)
RETURN src.id AS src_account_id, dst.id AS dst_account_id;

Result

Group variables

A graph pattern variable declared in a quantified pattern is considered a group variable when accessed outside the quantified pattern, and it binds to an array of matched graph elements.

You can access a group variable as an array where graph elements are preserved in the order of appearance along the matched paths. You can aggregate a group variable using horizontal aggregation.

Access group variable

In the following example, the variable e is accessed as the following:

• A graph pattern variable bound to a single edge in the WHERE clause e.amount > 100 (within the quantified pattern).
• A group variable bound to an array of edge elements in ARRAY_LENGTH(e) in the RETURN statement (outside the quantified pattern).
• A group variable bound to an array of edge elements, which is aggregated by SUM(e.amount) outside the quantified pattern. This is an example of horizontal aggregation.

GRAPH FinGraph
MATCH
  (src:Account {id: 7})-[e:Transfers WHERE e.amount > 100]->{0,2}
  (dst:Account)
WHERE src.id != dst.id
LET total_amount = SUM(e.amount)
RETURN
  src.id AS src_account_id, ARRAY_LENGTH(e) AS path_length,
  total_amount, dst.id AS dst_account_id;

Result

Any and Any Shortest paths

To limit the matched paths in each group of paths sharing the same source and destination nodes, you can use the ANY or ANY SHORTEST path search prefix. You can only apply these prefixes before an entire path pattern, and you can't apply them inside parentheses.

Match using ANY

The following query finds all reachable unique accounts which are one or two Transfers away from a given Account node.

The ANY path search prefix ensures that only one path between a unique pair of src and dst Account nodes is returned. In the following example, although you can reach the Account node with {id: 16} in two different paths from the source Account node, the results include only one path.

GRAPH FinGraph
MATCH ANY (src:Account {id: 7})-[e:Transfers]->{1,2}(dst:Account)
LET ids_in_path = ARRAY(SELECT e.to_id FROM UNNEST(e) AS e)
RETURN src.id AS src_account_id, dst.id AS dst_account_id, ids_in_path;

Result

Graph patterns

A graph pattern consists of one or more path patterns, separated by comma ,. Graph patterns can contain a WHERE clause, which lets you access all the graph pattern variables in the path patterns to form filtering conditions. Each path pattern produces a collection of paths.

Match using a graph pattern

The following query identifies intermediary accounts and their owners involved in transactions amounts exceeding 200, through which funds are transferred from a source account to a blocked account.

The following path patterns form the graph pattern:

• The first pattern finds paths where the transfer occurs from one account to a blocked account using an intermediate account.
• The second pattern finds paths from an account to its owning person.

The variable interm acts as a common link between the two path patterns, which requires interm to reference the same element node in both path patterns. This creates an equi-join operation based on the interm variable.

GRAPH FinGraph
MATCH
  (src:Account)-[t1:Transfers]->(interm:Account)-[t2:Transfers]->(dst:Account),
  (interm)<-[:Owns]-(p:Person)
WHERE dst.is_blocked = TRUE AND t1.amount > 200 AND t2.amount > 200
RETURN
  src.id AS src_account_id, dst.id AS dst_account_id,
  interm.id AS interm_account_id, p.id AS owner_id;

Result

Linear query statements

You can chain multiple graph statements together to form a linear query statement. The statements are executed in the same order as they appear in the query.

• Each statement takes the output from the previous statement as input. The input is empty for the first statement.
• The output of the last statement is the final result.

Find the maximum transfer to a blocked account

The following query finds the account and its owner with the largest outgoing transfer to a blocked account.

GRAPH FinGraph
MATCH (src_account:Account)-[transfer:Transfers]->(dst_account:Account)
WHERE dst_account.is_blocked
ORDER BY transfer.amount DESC
LIMIT 1
MATCH (src_account:Account)<-[owns:Owns]-(owner:Person)
RETURN src_account.id AS account_id, owner.name AS owner_name;

The following table illustrates how the intermediate results are passed along the statements. Only some properties of the intermediate results are shown, for brevity.

MATCH
  (src_account:Account)
    -[transfer:Transfers]->
  (dst_account:Account)
WHERE dst_account.is_blocked

ORDER BY transfer.amount DESC

LIMIT 1

MATCH
  (src_account:Account)
    <-[owns:Owns]-
  (owner:Person)

RETURN
  src_account.id AS account_id,
  owner.name AS owner_name

Result

Return statement

Return statement defines what to return from the matched patterns. It can access graph pattern variables, contain expressions and other clauses like ORDER_BY, GROUP_BY. See the RETURN statement.

Note that Spanner Graph doesn't support returning graph elements as query results. To return the entire graph element, use the TO_JSON function.

Return graph elements as JSON

GRAPH FinGraph
MATCH (n:Account {id: 7})
-- Returning a graph element in the final results is NOT allowed. Instead, use
-- the TO_JSON function or explicitly return the graph element's properties.
RETURN TO_JSON(n) AS n;

Result

Composing larger queries with NEXT keyword

You can chain multiple graph linear query statements together using the NEXT keyword. Input to the first linear query statement is empty. The output of each linear query statement becomes input to the next linear query statement.

The following example finds the owner of the account with the most incoming transfers by chaining multiple graph linear statements together. Note that you can use the same variable, account in this example, to refer to the same graph element across multiple linear statements.

GRAPH FinGraph
MATCH (:Account)-[:Transfers]->(account:Account)
RETURN account, COUNT(*) AS num_incoming_transfers
GROUP BY account
ORDER BY num_incoming_transfers DESC
LIMIT 1

NEXT

MATCH (account:Account)<-[:Owns]-(owner:Person)
RETURN account.id AS account_id, owner.name AS owner_name, num_incoming_transfers;

Result

Functions and expressions

You can use all GoogleSQL functions (both aggregate and scalar functions), operators, and conditional expressions in Spanner Graph query. Spanner Graph also supports graph-specific functions and operators.

Built-in functions and operators

The following functions and operators are commonly used in GQL:

• PROPERTY_EXISTS(n, birthday): Returns whether n exposes birthday property.
• LABELS(n): Returns the labels of n as defined in the graph schema.
• PROPERTY_NAMES(n): Returns the property names of n.
• TO_JSON(n): Returns n in JSON format. For more information, see the TO_JSON function.

The following query illustrates the PROPERTY_EXISTS predicate,LABELS function, and TO_JSON function, as well as other built-in functions like ARRAY_AGG and CONCAT.

GRAPH FinGraph
MATCH (person:Person)-[:Owns]->(account:Account)
RETURN person, ARRAY_AGG(account.nick_name) AS accounts
GROUP BY person

NEXT

RETURN
  LABELS(person) AS labels,
  TO_JSON(person) AS person,
  accounts,
  CONCAT(person.city, ", ", person.country) AS location,
  PROPERTY_EXISTS(person, is_blocked) AS is_blocked_property_exists,
  PROPERTY_EXISTS(person, name) AS name_property_exists
LIMIT 1;

Result

Subqueries

A subquery is a query nested in another query. The following lists Spanner Graph subquery rules:

• A subquery is enclosed within a pair of braces {}.
• A subquery might start with the leading GRAPH clause to specify the graph in scope. The specified graph doesn't need to be the same as the one used in the outer query.
• When the GRAPH clause is omitted in the subquery, the following occurs:
  • The graph in scope is inferred from the closest outer query context.
  • The subquery must start from a graph pattern matching statement with the MATCH.
• A graph pattern variable declared outside the subquery scope can't be declared again inside the subquery, but it can be referred to in expressions or functions inside the subquery.

Use a subquery to find the total number of transfers from each account

The following query illustrates the use of the VALUE subquery. The subquery is enclosed in braces {} prefixed by the VALUE keyword. The query returns the total amount of transfers initiated from an account.

GRAPH FinGraph
MATCH (p:Person)-[:Owns]->(account:Account)
RETURN p.name, account.id AS account_id, VALUE {
  MATCH (a:Account)-[transfer:Transfers]->(:Account)
  WHERE a = account
  RETURN SUM(transfer.amount) AS total_transfer
} AS total_transfer;

Result

For a list of supported subquery expressions, see Spanner Graph subqueries.

Query parameters

You can query Spanner Graph with parameters. For more information, see the syntax and learn how to query data with parameters in the Spanner client libraries.

The following query illustrates the use of query parameters.

GRAPH FinGraph
MATCH (person:Person {id: @id})
RETURN person.name;

Query graphs and tables together

You can use Graph queries in conjunction with SQL to access information from your Graphs and Tables together in a single statement.

GRAPH_TABLE

The GRAPH_TABLE operator takes a linear graph query and returns its result in a tabular form that can be seamlessly integrated into a SQL query. This interoperability lets you enrich graph query results with non-graph content and the other way around.

For example, you can create a CreditReports table and insert a few credit reports, as shown in the following example:

CREATE TABLE CreditReports (
  person_id     INT64 NOT NULL,
  create_time   TIMESTAMP NOT NULL,
  score         INT64 NOT NULL,
) PRIMARY KEY (person_id, create_time);

INSERT INTO CreditReports (person_id, create_time, score)
VALUES
  (1,"2020-01-10 06:22:20.222", 700),
  (2,"2020-02-10 06:22:20.222", 800),
  (3,"2020-03-10 06:22:20.222", 750);

Then, identify persons of interest through graph pattern matching in GRAPH_TABLE and join the graph query results with the CreditReports table to access credit score.

SELECT
  gt.person.id,
  credit.score AS latest_credit_score
FROM GRAPH_TABLE(
  FinGraph
  MATCH (person:Person)-[:Owns]->(:Account)-[:Transfers]->(account:Account)
  WHERE account.is_blocked
  RETURN DISTINCT person
) AS gt
JOIN CreditReports AS credit
  ON gt.person.id = credit.person_id
ORDER BY credit.create_time;

Result:

What's next

Learn best practices for tuning queries.