Overview

Constraints enforce rules for data that can go into a graph. Any data modification that violates a constraint will result in an error.

The following constraints are supported:

• EDGE KEY
• NOT NULL

Creating Constraints

A constraint is created with the CREATE CONSTRAINT statement. The constraint creation is executed as a job, you may run show().job() afterward to verify the success of the creation.

Creating a constraint can take some time, as the system needs to scan all existing data for compliance with the constraint. The creation process will fail if any existing data does not meet the constraint. Additionally, to ensure data consistency, other data modifications are put on hold during the constraint creation process.

EDGE KEY

The EDGE KEY constraint specifies an edge property effectively as the unique identifier for edges. Thus, the specified property must be present in all edge schemas with a consistent value type and must contain unique, non-null values across the graph.

Details

• A graph can have only one EDGE KEY constraint.
• The EDGE KEY constraint cannot be applied to an edge property of the list type.
• The edge property involved in the EDGE KEY constraint is automatically cached to accelerate query performance.
• The * must be used to apply the EDGE KEY constraint across all edge schemas.
• The EDGE KEY constraint creation enforces uniqueness among property values within each shard, allowing duplicates across different shards. However, any data modifications made after the EDGE KEY constraint is created must adhere to global uniqueness.

To create an EDGE KEY constraint named eUID on the edge property createdOn:

CREATE CONSTRAINT eUID
FOR ()-[e]-() REQUIRE e.createdOn IS EDGE KEY
OPTIONS {
  type: {createdOn: "datetime"}
}

This constraint can only be successfully created if the following conditions are met:

1. All edge schemas have a property named createdOn, and it must be of the type datetime.
2. The createdOn property must not contain duplicate values in each shard or null values.

Default Property Value Type

When the property value type is not specified, it defaults to string:

CREATE CONSTRAINT eUID
FOR ()-[e]-() REQUIRE e.createdOn IS EDGE KEY

In this case, all edge schemas must have a createdOn property of the type string.

Composite EDGE KEY Constraint

A composite EDGE KEY constraint involves multiple edge properties. The requirements remain the same as those for a single-property EDGE KEY, except that uniqueness is enforced based on the combined values of the specified properties.

To create a composite EDGE KEY constraint named eUIDs on the edge properties createdOn and weight:

CREATE CONSTRAINT eUIDs
FOR ()-[e]-() REQUIRE (e.createdOn, e.weight) IS EDGE KEY
OPTIONS {
  type: {createdOn: "datetime", weight: "float"}
}

NOT NULL

The NOT NULL constraint enforces a property cannot have null values, ensuring that a value is always provided.

Details

• A NOT NULL constraint must be created for a single specified schema.
• Only one property can be designated per NOT NULL constraint.

To create a NOT NULL constraint named nn_1 on the property name of the user nodes:

CREATE CONSTRAINT nn_1
FOR (u:user) REQUIRE u.name IS NOT NULL

To create a NOT NULL constraint named nn_2 on the property weight of the link edges:

CREATE CONSTRAINT nn_2
FOR ()-[e:link]-() REQUIRE e.weight IS NOT NULL

These constraints can only be successfully created when there is no null values exist in the specified property.

Using the IF NOT EXISTS Flag

Constraint names in a graph must be unique. If you attempt to create a constraint with a name that already exists, the creation will fail, and an error message will indicate the duplication.

With the IF NOT EXISTS flag, the job will complete with a FINISHED status when a duplicate name is detected. No error message will be returned, and no new constraint is created.

To create a NOT NULL constraint named nn_1. If the constraint name already exists, skip the creation without returning an error message:

CREATE CONSTRAINT nn_1
FOR (u:user) REQUIRE u.age IS NOT NULL

Showing Constraints

To retrieve information about constraints created in the current graph:

// Shows all constraints
show().constraint()

// Shows all constraints created on node properties
show().node_constraint()

// Shows all constraints created on edge properties
show().edge_constraint()

The information about constraints is organized into the _nodeConstraint or _edgeConstraint table. Each table includes fields that provide essential details about each constraint:

Dropping Constraints

A constraint can be dropped with the DROP CONSTRAINT statement.

To drop a constraint named nn_1:

DROP CONSTRAINT nn_1

Using the IF EXISTS Flag

The constraint deletion will fail and output an error message if the specified constraint name does not exist.

With the IF EXISTS flag, no error message will be returned when the specified constraint name is not found, and no constraint is deleted.

To drop a constraint named nn. If the constraint name does not exist, skip the deletion without returning an error message:

DROP CONSTRAINT nn_1 IF EXISTS

Restrictions on Properties with Constraints

Renaming Properties

Properties with the NOT NULL constraints can be renamed. However, renaming properties with an EDGE KEY constraint is not allowed.

Dropping Properties

A property with a constraint cannot be dropped until all the related constraints are deleted.