PostgreSQL · January 2, 2024

PostgreSQL Command: CREATE RULE

PostgreSQL Command: CREATE RULE

PostgreSQL is a powerful open-source relational database management system that offers a wide range of features and functionalities. One of the essential commands in PostgreSQL is the CREATE RULE command, which allows users to define custom rules for handling data manipulation operations.

What is a Rule in PostgreSQL?

In PostgreSQL, a rule is a named object that defines an action to be taken when a specified event occurs. These events can include INSERT, UPDATE, DELETE, or SELECT operations on a table or view. Rules are often used to implement complex business logic or enforce data integrity constraints.

Syntax of CREATE RULE Command

The syntax for the CREATE RULE command in PostgreSQL is as follows:

CREATE [ OR REPLACE ] RULE rule_name AS ON event
    TO table_name
    [ WHERE condition ]
    DO [ ALSO | INSTEAD ] { NOTHING | command }

Let's break down the different components of this syntax:

  • OR REPLACE: This optional clause allows you to replace an existing rule with the same name.
  • rule_name: The name of the rule you want to create.
  • event: The event that triggers the rule. It can be one of INSERT, UPDATE, DELETE, or SELECT.
  • table_name: The name of the table or view on which the rule is defined.
  • condition: An optional condition that must be satisfied for the rule to be triggered.
  • ALSO | INSTEAD: Specifies whether the rule should be executed in addition to or instead of the original command.
  • NOTHING | command: The action to be taken when the rule is triggered. It can be either doing nothing or executing a specific command.

Example

Let's consider an example where we want to create a rule that prevents users from deleting records from a table called "employees" if their salary is above a certain threshold. Here's how we can achieve this:

CREATE RULE prevent_delete_high_salary AS ON DELETE TO employees
    WHERE old.salary > 10000
    DO INSTEAD NOTHING;

In this example, the rule named "prevent_delete_high_salary" is created on the DELETE event for the "employees" table. The rule specifies a condition where the salary of the record being deleted should be greater than 10,000. If this condition is met, the rule will be triggered, and instead of executing the DELETE command, it will do nothing.

Summary

The CREATE RULE command in PostgreSQL allows users to define custom rules for handling data manipulation operations. Rules are powerful tools that enable the implementation of complex business logic and data integrity constraints. By using the CREATE RULE command effectively, users can enhance the functionality and control of their PostgreSQL databases.

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