> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify-poc.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Ensure data integrity with constraints

> To prevent invalid data, apply rules to your table columns using constraints.

export const CHUNK = 'chunk';

export const HYPERTABLE = 'hypertable';

export const PG = 'Postgres';

Constraints are rules that apply to your database columns. This prevents you from entering invalid data into your
database. When you create, change, or delete constraints on your {HYPERTABLE}s, the constraints are propagated to the
underlying {CHUNK}s, and to any indexes.

{HYPERTABLE}s support all standard {PG} constraint types. For foreign keys in particular, the following is supported:

* Foreign key constraints from a {HYPERTABLE} referencing a regular table
* Foreign key constraints from a regular table referencing a {HYPERTABLE}

Foreign keys from a {HYPERTABLE} referencing another {HYPERTABLE} **are not supported**.

For example, you can create a table that only allows positive device IDs, and non-null temperature readings. You can
also check that time values for all devices are unique. To create this table, with the constraints, use this command:

```sql theme={"dark"}
CREATE TABLE conditions (
    time       TIMESTAMPTZ
    temp       FLOAT NOT NULL,
    device_id  INTEGER CHECK (device_id > 0),
    location   INTEGER REFERENCES locations (id),
    PRIMARY KEY(time, device_id)
) WITH (
    tsdb.hypertable
);
```

This example also references values in another `locations` table using a foreign key constraint.

<Info>
  Time columns used for partitioning must not allow `NULL` values. A `NOT NULL` constraint is added by default to these
  columns if it doesn't already exist.
</Info>

For more information on how to manage constraints, see the [{PG} docs][postgres-createconstraint].

[postgres-createconstraint]: https://www.postgresql.org/docs/current/ddl-constraints.html