Constraints

In modern database systems, ensuring data integrity and reducing redundancy is paramount. Constraints play a pivotal role, ensuring that only valid data is entered into the database upon commit. The following chapters delve deep into two fundamental types of constraints, existence and uniqueness.

The existence constraint ensures the presence of specific data within the database, while the uniqueness constraint ensures that specific label-property pairs remain unique across entries.

Existence constraint

Existence constraint enforces that each node with a specific label must also have a certain property. Only one label and property can be supplied at a time.

This constraint can be enforced using the following language construct:

CREATE CONSTRAINT ON (n:label) ASSERT EXISTS (n.property);

To confirm that the constraint was successfully created use the following query:

SHOW CONSTRAINT INFO;

Trying to modify the database in a way that violates the constraint will yield an error.

Constraints are dropped using the DROP clause:

DROP CONSTRAINT ON (n:label) ASSERT EXISTS (n.property);

Example

If the database is used to hold basic employee information, each employee should have a first name and a last name. You can enforce this by running the following queries:

CREATE CONSTRAINT ON (n:Employee) ASSERT EXISTS (n.first_name);
CREATE CONSTRAINT ON (n:Employee) ASSERT EXISTS (n.last_name);

The SHOW CONSTRAINT INFO; should return the following result:

+-----------------+-----------------+-----------------+
| constraint type | label           | properties      |
+-----------------+-----------------+-----------------+
| exists          | Employee        | first_name      |
| exists          | Employee        | last_name       |
+-----------------+-----------------+-----------------+

To drop the created constraints use the following queries:

DROP CONSTRAINT ON (n:Employee) ASSERT EXISTS (n.first_name);
DROP CONSTRAINT ON (n:Employee) ASSERT EXISTS (n.last_name);

Now, SHOW CONSTRAINT INFO; returns an empty set.

Uniqueness constraint

The uniqueness constraint enforces that each label-property pair is unique. You can also, specify multiple properties when creating uniqueness constraints.

The uniqueness constraint can be enforced using the following language construct:

CREATE CONSTRAINT ON (n:label) ASSERT n.property1, n.property2, ..., IS UNIQUE;

To confirm that the constraint was successfully created use the following query:

SHOW CONSTRAINT INFO;

Trying to modify the database in a way that violates the constraint will yield an error Unable to commit due to unique constraint violation on :Label(property).

Constraints are dropped using the DROP clause:

DROP CONSTRAINT ON (n:label) ASSERT n.property IS UNIQUE;

Example

If the database is used to hold basic employee information, each employee should have a unique id and email. You can enforce this by running the following query:

CREATE CONSTRAINT ON (n:Employee) ASSERT n.id IS UNIQUE;
CREATE CONSTRAINT ON (n:Employee) ASSERT n.email IS UNIQUE;

The SHOW CONSTRAINT INFO; should return the following result:

+-----------------+-----------------+-----------------+
| constraint type | label           | properties      |
+-----------------+-----------------+-----------------+
| unique          | Employee        | id              |
| unique          | Employee        | email           |
+-----------------+-----------------+-----------------+

To specify multiple properties when creating uniqueness constraints, list them one after the other:

CREATE CONSTRAINT ON (n:Employee) ASSERT n.name, n.address IS UNIQUE;

At this point, SHOW CONSTRAINT INFO; yields the following result:

+-----------------+-----------------+-----------------+
| constraint type | label           | properties      |
+-----------------+-----------------+-----------------+
| unique          | Employee        | id              |
| unique          | Employee        | email           |
| unique          | Employee        | name, address   |
+-----------------+-----------------+-----------------+

This means that two employees could have the same name or the same address, but they can not have the same name and the same address.

To drop the created constraints, use the following queries:

DROP CONSTRAINT ON (n:Employee) ASSERT n.id IS UNIQUE;
DROP CONSTRAINT ON (n:Employee) ASSERT n.email IS UNIQUE;
DROP CONSTRAINT ON (n:Employee) ASSERT n.name, n.address IS UNIQUE;

Now, SHOW CONSTRAINT INFO; returns an empty set.

Data type constraint

The data type constraint enforces that each label-property pair is of a certain data type.

The data type constraint can be enforced using the following language construct:

CREATE CONSTRAINT ON (n:label) ASSERT n.property IS TYPED DATA_TYPE;

The supported data types are:

To confirm that the constraint was successfully created use the following query:

SHOW CONSTRAINT INFO;

Trying to modify the database in a way that violates the constraint will yield an error IS TYPED DATA_TYPE violation on Label(property).

Constraints are dropped using the DROP clause:

DROP CONSTRAINT ON (n:label) ASSERT n.property IS TYPED DATA_TYPE;

You can only have one data type constraint on a given label-property pair. Attempting to create a second data type constraint on a given label-property pair will yield an error Constraint IS TYPED DATA_TYPE on :Label(property) already exists. Attempting to drop a data type constraint which doesn't exist will yield an error Constraint IS TYPED DATA_TYPE on :Node(prop) doesn't exist.

Example

If the database is used to hold basic information about a person like their name and age you can enforce the name to be a string and the age to be an integer by running the following queries:

CREATE CONSTRAINT ON (n:Person) ASSERT n.name IS TYPED STRING;
CREATE CONSTRAINT ON (n:Person) ASSERT n.age IS TYPED INTEGER;

Then SHOW CONSTRAINT INFO; should return the following result:

+-----------------+--------------+-----------------+-------------+
| constraint type | label        | properties      |  data_type  |
+-----------------+--------------+-----------------+-------------|
| data_type       | Person       | name            |   STRING    |
| data_type       | Person       | age             |   INTEGER   |
+-----------------+--------------+-----------------+-------------|

Creating a person with

CREATE (:Person {age:22});

and trying to violate the data type constraints by setting the age of a person to a string with:

MATCH (n) SET n.age = 'age';

will yield the error IS TYPED INTEGER violation on Person(age).

To drop the created constraints, use the following queries:

DROP CONSTRAINT ON (n:Person) ASSERT n.name IS TYPED STRING;
DROP CONSTRAINT ON (n:Person) ASSERT n.age IS TYPED INTEGER;

Now, SHOW CONSTRAINT INFO; returns an empty set.

Schema-related procedures

You can also modify the constraints using the schema.assert() procedure.

Delete all constraints

To delete all constraints, use the schema.assert() procedure with the following parameters:

• indices_map = map of key-value pairs of all indexes in the database
• unique_constraints = {}
• existence_constraints = {}
• drop_existing = true

Here is an example of indexes and constraints set in the database:

CREATE CONSTRAINT ON (n:Person) ASSERT EXISTS (n.name);
CREATE CONSTRAINT ON (n:Employee) ASSERT n.id IS UNIQUE;
CREATE CONSTRAINT ON (n:Employee) ASSERT n.email IS UNIQUE;
CREATE CONSTRAINT ON (n:Employee) ASSERT n.name, n.surname IS UNIQUE;
CREATE INDEX ON :Student(id);
CREATE INDEX ON :Student;

There are three uniqueness and one existence constraint. Additionally, there are two indexes - one label and one label-property index. To delete all constraints, run:

CALL schema.assert({Student: ["", "id"]}, {}, {}, true) 
YIELD action, key, keys, label, unique
RETURN action, key, keys, label, unique;

The above query removes all existing constraints because the empty unique_constraints and existence_constraints maps indicate that no constraints should be asserted as existing, while the drop_existing set to true specifies that all existing constraints should be dropped.

Primarily, the assert() procedure is used to define a schema, but it's also useful if you need to delete all node indexes or delete all node indexes and constraints.

Recovery

Existence and unique constraints, and indexes can be recovered in parallel. To enable this behavior, set the storage-parallel-schema-recovery configuration flag to true.