Constraints

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

Showing Constraints

To retrieve all constraints created on node properties in the current graph:

SHOW NODE CONSTRAINT

To retrieve all constraints created on edge properties in the current graph:

SHOW EDGE CONSTRAINT

The information about constraints is organized into the _nodeConstraint or _edgeConstraint table, including the following fields:

Creating Constraints

A constraint is created with the ALTER NODE or ALTER EDGE statement. The constraint creation is executed as a job, you may run SHOW JOB <id?> 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

An EDGE KEY constraint designates either a single edge property or a combination of edge properties as the unique identifier for all edges in the graph. When multiple properties are specified, it is also referred to as a composite EDGE KEY.

Each property in the EDGE KEY must be present in every edge schema and use a consistent value type. For a single-property EDGE KEY, that property must contain unique, non-null values across all edges. For a composite EDGE KEY, each property must be non-null, and the combination of their values must be unique across all edges.

Details

• A graph can have only one EDGE KEY - either single-property or composite.
• Properties of the list type cannot be used in the EDGE KEY.
• Properties involved in the EDGE KEY are automatically cached to accelerate query performance.
• When the EDGE KEY is created, uniqueness is enforced within each shard. Duplicates may exist across shards at creation time, but all subsequent data modifications must comply with global uniqueness.

To create an EDGE KEY constraint on the edge property eID. This constraint can only be successfully created if the following conditions are met:

1. All edge schemas have a property named eID, and it must be of the type int32.
2. The eID property must not contain duplicate values in each shard or null values.

ALTER EDGE * ADD CONSTRAINT EDGE KEY ON eID INT32

Default Property Value Type

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

ALTER EDGE * ADD CONSTRAINT EDGE KEY ON tag

In this case, all edge schemas must have a tag 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 on the edge properties eID and tag:

ALTER EDGE * ADD CONSTRAINT EDGE KEY ON eID INT32, tag STRING

NOT NULL

The NOT NULL constraint enforces a property cannot have null values, ensuring that a value is always provided. Note that a NOT NULL constraint can be defined only on a single property of a single schema.

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

ALTER NODE user ADD CONSTRAINT NOT NULL ON name

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

ALTER EDGE link ADD CONSTRAINT NOT NULL ON weight

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

Using IF NOT EXISTS

The IF NOT EXISTS clause is used to prevent errors when attempting to create a constraint that already exists. It allows the statement to be safely executed.

ALTER NODE user ADD CONSTRAINT IF NOT EXISTS NOT NULL ON name

This creates the constraint only if there is no existing NOT NULL constraint on the name property of user nodes. If such a constraint already exists, the statement is ignored without throwing an error.

Dropping Constraints

A constraint can be dropped with the ALTER NODE or ALTER EDGE statement.

To drop the EDGE KEY constraint from the current graph:

ALTER EDGE * DROP EDGE KEY

To drop the NOT NULL constraint on the name property of user nodes:

ALTER NODE user DROP CONSTRAINT NOT NULL ON name

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.