conditional_execution

Queries might require conditional execution logic that can’t be adequately expressed in Cypher. The do module makes it possible to define complex logic that will control query execution.

Procedures

The following procedures can’t be used to run queries that execute global operations:

• creating and deleting indexes
• creating and deleting constraints
• changing the global isolation level
• setting the storage mode

case()

When given a list of condition-query pairs, do.case executes the query associated with the first condition evaluating to true (or the else query if none are true) with the given parameters.

Parameters are prefixed with $ like $param_name. Check the documentation on querying for examples.

Input:

• conditionals: List[Any] ➡ Variable-length list of condition-query pairs structured as [condition, query, condition, query, …​]. Conditions are boolean and queries are string.
• else_query: string (default = "") ➡ The query to be executed if no condition evaluates to true.
• params: Map (default = NULL) ➡ If any of the given queries is parameterized, provide a {param_name: param_value} map to be applied to them.

Output:

• value: Map ➡ Contains the result record of the executed query. Each value corresponds to one result record.

Usage:

The following example checks how many nodes there are in the database, and returns empty if there are 0 nodes, one node if there is 1 node, or multiple nodes if any other result is produced by the size(collect(n)) functions:

MATCH (n)
WITH size(collect(n)) as n_nodes
CALL do.case([n_nodes = 0,
              "RETURN 'empty' AS graph_status;",
              n_nodes = 1,
              "RETURN 'one node' AS graph_status;"],
              "RETURN 'multiple nodes' AS graph_status;")
YIELD value
RETURN value.graph_status AS graph_status;

when()

do.when evaluates the given condition and executes the if query or the else query depending on whether the condition is satisfied.

Parameters are prefixed with $ like $param_name.

Input:

• condition: boolean ➡ Determines what query to execute.
• if_query: string ➡ The query to be executed if the condition is satisfied.
• else_query: string (default = "") ➡ The query to be executed if the condition isn’t satisfied.
• params: Map (default = NULL) ➡ If if_query or else_query are parameterized, provide a {param_name: param_value} map to be applied.

Output:

• value: Map ➡ Contains the result record of the executed query. Each value corresponds to one result record.

Usage:

The following example checks how many nodes there are in the database, checks if the result is 0, and returns empty if is, and not empty if it isn’t:

MATCH (n)
WITH size(collect(n)) as n_nodes
CALL do.when(n_nodes = 0,
             "RETURN 'empty' AS graph_status;",
             "RETURN 'not empty' as graph_status;")
YIELD value
RETURN value.graph_status AS graph_status;

Example

MERGE (a:Node {id: 0}) MERGE (b:Node {id: 1}) CREATE (a)-[:RELATION]->(b);
MERGE (a:Node {id: 1}) MERGE (b:Node {id: 2}) CREATE (a)-[:RELATION]->(b);
MERGE (a:Node {id: 0}) MERGE (b:Node {id: 2}) CREATE (a)-[:RELATION]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 4}) CREATE (a)-[:RELATION]->(b);
MERGE (a:Node {id: 3}) MERGE (b:Node {id: 5}) CREATE (a)-[:RELATION]->(b);
MERGE (a:Node {id: 4}) MERGE (b:Node {id: 5}) CREATE (a)-[:RELATION]->(b);

MATCH (n:Node)
WITH size(collect(n)) as n_nodes
CALL do.when(n_nodes = 0,
             "RETURN 'empty' AS graph_status;",
             "RETURN 'not empty' as graph_status;")
YIELD value
RETURN value.graph_status AS graph_status;

┌──────────────┐
│ graph_status │
├──────────────┤
│ not empty    │ 
└──────────────┘