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123 lines
3.3 KiB
ReStructuredText
123 lines
3.3 KiB
ReStructuredText
.. meta::
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:description: Use Postgres constraints with Hasura
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:keywords: hasura, docs, postgres, constraints
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.. _postgres_constraints:
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Postgres constraints
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====================
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.. contents:: Table of contents
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:backlinks: none
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:depth: 2
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:local:
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Introduction
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------------
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`Postgres constraints <https://www.postgresql.org/docs/current/ddl-constraints.html>`__ are used to define rules for columns in a database table. They ensure that
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no invalid data is entered into the database.
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.. note::
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For more detailed information on Postgres constraints, please refer to the `Postgres documentation <https://www.postgresql.org/docs/current/ddl-constraints.html>`__.
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Postgres constraints
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--------------------
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There are different types of constraints that can be used with Postgres.
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Primary key constraints
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^^^^^^^^^^^^^^^^^^^^^^^
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A ``PRIMARY KEY`` is used to identify each row of a table uniquely.
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**Identify the author's id as the primary key of the authors table:**
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.. code-block:: sql
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:emphasize-lines: 2
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CREATE TABLE authors(
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id INT PRIMARY KEY,
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name TEXT NOT NULL
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);
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Foreign key constraints
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^^^^^^^^^^^^^^^^^^^^^^^
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A foreign key constraint specifies that the values in a column must match the values appearing in a row of another table.
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Foreign key constraints are used to create relationships between tables.
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**Define the author_id in the articles table as a foreign key to the id column in the authors table:**
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.. code-block:: sql
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:emphasize-lines: 11
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CREATE TABLE authors(
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id SERIAL PRIMARY KEY,
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name TEXT NOT NULL,
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email TEXT UNIQUE
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);
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CREATE TABLE articles(
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id SERIAL PRIMARY KEY,
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title TEXT NOT NULL,
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author_id INTEGER,
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FOREIGN KEY (author_id) REFERENCES authors (id)
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);
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Not-null constraints
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^^^^^^^^^^^^^^^^^^^^
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A not-null constraint allows you to specify that a column's value cannot be ``null``.
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**Validate that an author's name cannot be null:**
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.. code-block:: sql
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:emphasize-lines: 2-3
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CREATE TABLE authors(
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id SERIAL PRIMARY KEY,
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name TEXT NOT NULL
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);
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Unique constraints
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^^^^^^^^^^^^^^^^^^
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Unique constraints prevent database entries with a duplicate value of the respective column.
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**Validate that an author's email is unique:**
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.. code-block:: sql
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:emphasize-lines: 4
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CREATE TABLE authors(
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id SERIAL PRIMARY KEY,
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name TEXT NOT NULL,
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email TEXT UNIQUE
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);
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Check constraints
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^^^^^^^^^^^^^^^^^
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Check constraints allow you to specify a ``Boolean`` expression for one or several columns.
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This Boolean expression must be satisfied (equal to ``true``) by the column value for the object to be inserted.
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**Validate that an author's rating is between 1 and 10:**
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.. code-block:: sql
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:emphasize-lines: 4
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CREATE TABLE authors(
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id SERIAL PRIMARY KEY,
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name TEXT NOT NULL,
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rating INT NOT NULL CHECK(rating > 0 AND rating <= 10)
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);
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Postgres constraints & Hasura
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-----------------------------
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Most Postgres constraints (primary key, foreign key, not-null and unique constraints) can be added to Hasura natively when :ref:`creating tables <create_tables>`.
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Postgres check constraints can be used as a form of data validation in Hasura and can be added :ref:`as described here <data_validations_check_constraints>`.
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