What is a view in a relational database?

A view in a relational database is a virtual table created by a saved SQL query. Unlike physical tables, views don’t store data themselves but instead dynamically retrieve and present data from one or more underlying tables whenever they’re accessed. Views act as predefined filters or transformations, allowing users to interact with a subset of data or a combined representation of data without modifying the original tables. For example, a view could combine columns from a customers table and an orders table to show a customer’s name alongside their recent purchases, all while keeping the source tables unchanged.

Views are primarily used to simplify complex queries, enforce security, and abstract underlying schema details. For instance, a developer might create a view named active_users that filters a users table to include only rows where last_login is within the past 30 days. This allows other developers to query active_users without rewriting the filter logic repeatedly. Views also enhance security by restricting access to sensitive columns. A database administrator could create a view that exposes only non-sensitive fields (e.g., product_name and price) from a products table, while hiding internal details like cost_price or supplier_id. This ensures users interact with the data they’re authorized to see.

However, views have limitations. Since they execute their underlying query each time they’re accessed, complex views with joins or aggregations can impact performance. For example, a view that calculates total sales per region by joining orders, customers, and locations tables might slow down if the tables grow large. Additionally, not all views are updatable—this depends on the database system and the view’s structure. A view that aggregates data (e.g., monthly_sales) generally can’t be modified directly because the database can’t map changes back to the source tables. Developers should use views judiciously, balancing convenience with performance and maintainability.