Views in PostgreSQL are virtual tables that are generated based on the result of a query. They provide a way to present data from one or more tables in a structured format. Views are useful for simplifying complex queries, providing an additional layer of security, and encapsulating complex logic.

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Creating a View

To create a view in PostgreSQL, you use the CREATE VIEW statement followed by the view name and the query that defines the view. For example, assuming you have two tables employees and departments (we created them in the tables section of this guide), you can create a view that combines data from both tables: 
CREATE VIEW employee_details AS
SELECT employees.employee_id,
       employees.first_name,
       employees.last_name,
       departments.department_name
FROM employees
JOIN departments ON employees.department_id = departments.department_id;
This view provides an abstraction over the underlying tables and allows you to query the combined data without having to write the full join every time. You can query the view just like a regular table: 
SELECT * FROM employee_details;

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Updating a View

There are two ways to update a view in PostgreSQL:

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Recreating the View

You can update a view by replacing it with the updated query. This allows you to modify the underlying query, as long as the column names and data types of existing columns remain the same and in the same order. You can add new columns to the end of the SELECT statement, and also modify the underlying logic behind the columns as long as the name and type remain the same. Because the view does not contain any data itself, replacing it is a fast, atomic and idempotent (repeatable) operation. Because the limitations prevent you from making any breaking changes, the operation is safe to perform online (while the application is running). For example, if you want to include the employee email in the employee_details view, you can replace the view as follows: 
CREATE OR REPLACE VIEW employee_details AS
SELECT employees.employee_id,
       employees.first_name,
       employees.last_name,
       departments.department_name,
       employees.email
FROM employees
JOIN departments ON employees.department_id = departments.department_id;
The email column is added to the view definition, and the view is replaced with the updated query. We have to add the new column as the last column in the SELECT statement to avoid breaking changes.

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Altering a View

Some properties of a view can be altered without replacing the view definitions, for example you can change a view name, owner, rename a column or modify default values. For example: 
ALTER VIEW employee_details RENAME COLUMN department_name TO dept_name;

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Dropping a View

To drop a view in PostgreSQL, you use the DROP VIEW statement followed by the view name: 
DROP VIEW employee_details;
Dropping a view and re-creating it is also the way to modify a view in ways that are not supported by the CREATE OR REPLACE VIEW syntax. You can do this in a single transaction to ensure that the view is always available for querying, but be aware that if you remove columns that are still in use, you may break the application. 
BEGIN;
DROP VIEW employee_details;
-- note that the below view definition has different columns from the original view
CREATE VIEW employee_details AS
SELECT employees.employee_id,
       employees.first_name || ' ' || employees.last_name as full_name,
       employees.email,
       departments.department_name
FROM employees
JOIN departments ON employees.department_id = departments.department_id;
COMMIT;