Illustration showing SQL views management with examples, use cases and best practices for effective database design.

6.How do you handle views in multiple schemas?
Managing views across multiple schemas requires careful planning and organization. Here are the essential strategies:
1.Centralized View Management
There should be a schema especially created to hold all views shared. This makes central management and maintenance much easier.
Use clear naming conventions. Views should easily identify what they are about through consistent naming conventions. Document in detail for each view regarding its purpose, underlying tables, and usage guidelines.
2.Permission Grant:
•Role-Based Access Control: Role or user assigned appropriate permissions to control which views to access.
•Fine-Grained Permissions: Specific permissions for viewing and querying data but modification privileges were restricted to the authorized users
3.Schema-Specific Views:
•Local Views: Views were created within individual schemas to encapsulate specific data and logic for that schema.
•Cross-schema references: SCHEMA qualifier when a reference to an object is necessary in another schema
4.Periodical Review and Maintenance
•Review period: Periodically scan the views for correctness, as well as relevance in a system.
•Update Period: Update the views whenever needed either from changes in its underlying tables or due to new business requirements.
•Drop unused views: One deletes unused views to avoid clutter and can possibly impact performance.
With the above guidance, you will be dealing with more than one schema and guarantee that your data is consistent, secure and accessible.

7.How do you use the WITH CHECK OPTION when you are creating a view so that integrity can be ensured about your data?
The clause with check option is surely very powerful in maintaining data integrity in SQL views. But while attached with the clause of CREATE VIEW, this surely makes sure that there should always be a condition to follow the defined rule that is present in view itself for the data modifications to occur.
For example, we have a table of Products with columns ProductID, ProductName and Price. We can then define a view called ExpensiveProducts which will only return products whose prices are above 100:
CREATE VIEW ExpensiveProducts
AS
SELECT ProductID, ProductName, Price
FROM Products
WHERE Price > 100
WITH CHECK OPTION;
Now, if trying to update a product price from the ExpensiveProducts view to a value less than 100 is not accepted by the database because the update violates the WHERE clause of the view, this would prevent data corruption caused by accident or even by bad intention and thus preserve the integrity of the data.
Worth noting is the fact that WITH CHECK OPTION clause is not supported in all database systems. Another thing is the effect of complexity of the view and of the structures in the tables underlying the view. Views that are quite complex could make this clause inapplicable.
Sensible usage of the WITH CHECK OPTION clause can greatly strengthen data integrity and security for your database system.

8.How would you use operations on a view? Such as SELECT,INSERT,UPDATE,DELETE.
Operations on Views
Despite the fact that views make the access and modifications of the data easier, this capability is applied only with respect to the definition of a view or database system applied.
SELECT:
•Directly Query the View: Just like you can query any table using a SELECT, you can query a view directly using a SELECT.
INSERT, UPDATE and DELETE :
•Updatable Views : For some types of views it is possible to do an INSERT, UPDATE or DELETE upon the view. However that must be possible on its underlying base tables.
•Restrictions: Generally, views which contain complex joins, aggregate functions, or GROUP BY clauses are not updatable.
•INSTEAD OF Triggers: In a few database systems, you can use INSTEAD OF triggers to specify special actions to be performed in case you perform INSERT, UPDATE or DELETE operations on a view. This helps you control how data in the underlying tables is modified.
Important Points:
•View Definition: The definition of the view-complexity, including joins, aggregate functions and subqueries-may affect its updatability.
•Base Tables: Constraints or triggers applied to the base tables to limit modifications to them will be propagated to the view.
•Database System: Support for operations against views varies from one DB system to another. Refer to the documentation for your specific database system for more details on how it handles these. Understanding these factors and the limitations of views, one can make adequate use of views to further ease data access and its manipulation without compromising the data integrity.

9.How do you manage complexity in queries when building views for simplification of operations?
Handling complex queries when creating views simplifies repetitive tasks and makes the work with data easier. A view is a kind of virtual table that stores a result of a predefined query. Instead of writing and then managing a complex query in a loop, you might define it once as a view and use it afterwards like a table.
For example, to make a sum of the sales for every customer based on the joining Customers and Orders, the query could contain join and aggregate operation as well:
CREATE VIEW CustomerSales AS
SELECT
c.CustomerID,
c.Name AS CustomerName,
SUM(o.OrderAmount) AS TotalSales
FROM
Customers c
INNER JOIN
Orders o ON c.CustomerID = o.CustomerID
GROUP BY
c.CustomerID, c.Name;
This view encapsulates the logic of calculation of total sales. Now, instead of re-writing a query every time, one could just query the view. Suppose one wants to know all customers with sales greater than 1000.
SELECT * FROM CustomerSales WHERE TotalSales > 1000;
This reduces code duplication and makes the operations simple to the end user or developers. The views are specifically useful in reporting and analytics where complex calculations and joins are quite common.

10.How to update a view if the underlying table structure changes?
If the structure of an underlying table changes, then you need to update a view in order to make it still compatible with the new structure. Here is how to do it:
1. Identify Changes in the Table: Determine what has been changed in the table structure, that is, columns added, removed or renamed. Such changes may impact columns used in the view.
2. Update the View : You update the definition of the view using the CREATE OR REPLACE VIEW statement. It allows you to change the view's query based on the new structure of the table.
Suppose that the column called OrderDate in the Orders table was there earlier but its name was changed to be known as PurchaseDate. Its updated view also requires this change:
Earlier View
CREATE VIEW RecentOrders AS
SELECT OrderID, OrderDate FROM Orders WHERE OrderDate > '2024-01-01';
Updated View
CREATE OR REPLACE VIEW RecentOrders AS
SELECT OrderID, PurchaseDate FROM Orders WHERE PurchaseDate > '2024-01-01';
3.Test the View: You should update the script and run the query as a view to verify functionality.
4.Drop and Recreate (if necessary): Your database won't have support for using CREATE OR REPLACE VIEW drop the previous view created to use the new updated queries.
5. Update dependencies: Check and update all the dependent queries, reports or applications using the view to ensure they process changes properly.
This will make sure that your view is functional and also updated with the new table structure.

11.How do you use views for security purposes to control access to sensitive data?
Views are some strong SQL tools when talking about data security. They can be useful when you would like to have control over a table by displaying just the selected columns or rows while hiding important data, say your salary or address. You might, for example, have an Employees table where there are some columns as sensitive as the Salary column or SSN number. You may want a view with those columns masked out.
CREATE VIEW PublicEmployeeData AS
SELECT EmployeeID, Name, Department FROM Employees;
You can grant the users access to this view while denying them access to the underlying table. That way, the users would only be exposed to necessary data without exposing sensitive information. Views also enforce consistent security policies and simplify permission management.

12.How do you use JOIN in a view to join data from multiple tables?
In SQL views you can create a virtual table that spans two or more tables based on the type of JOIN used. Hence, instead of using individual queries you can easily get multiple related information with a single query.
Here's an example of having an Employees and Departments: a view is created with employee information appended to a department name like this :
CREATE VIEW EmployeeDepartment AS
SELECT e.EmployeeID, e.Name, d.DepartmentName
FROM Employees e
INNER JOIN Departments d ON e.DepartmentID = d.DepartmentID;
In the example, it puts together employee information and corresponding department name by using Employees and Departments tables with joining it upon DepartmentID. A view can be queried just as if it were one table, making it easy for users to access related information.

13.How do I create a view that involves an aggregate function such as SUM() or AVG()?
You will end up getting the view containing an aggregate function like SUM() or AVG() on your database. That means complex calculations can be made through a simplified view and hence it improves the way data analysis is achieved. You can do all complex calculations on huge data sets without re-writing the same logic in all the queries by using aggregate functions like SUM(), AVG(), COUNT(), etc. While defining a view with aggregate functions, you generally use GROUP BY clause to group your data by some columns and then apply the aggregate function to get a summary of the data. For example, suppose that you have a table called Sales with columns ProductID and SaleAmount. Suppose you need to know the total sales for each product. You can create a view so that you do not repeat this computation in every query.
Here is how
CREATE VIEW TotalSalesPerProduct AS
SELECT ProductID, SUM(SaleAmount) AS TotalSales
FROM Sales
GROUP BY ProductID;
In this example, the view TotalSalesPerProduct aggregates SaleAmount by ProductID for the computation of total sales concerning each product. Since GROUP BY is applied by ProductID the output will be a sum taken per each product. After you have built a view, you easily access this sum using the following select statement: SELECT * FROM TotalSalesPerProduct;
This makes your queries simpler because you are not required to make constant recalculations of your total sales each time you need this information. This approach gives you a reliable, reusable way of retrieving your aggregated data.
Use views with aggregate functions since these have the advantage of calculating once and consistently throughout all your queries. These can be especially very handy when you have reporting or analysis where you might summarize the data quite often.

14.How do you use a view to present summarized or formatted data to users?
To use a view for displaying summarized or formatted data to users, you aggregate, filter and format the data as needed. Views enable you to hide the complexity of underlying queries and present the data in a more user-friendly and understandable format. For example, you could use a view to show the total sales for each product from a Sales table:
CREATE VIEW SalesSummary AS
SELECT ProductID, ProductName, SUM(SaleAmount) AS TotalSales
FROM Sales
GROUP BY ProductID, ProductName;
In this view, the calculation has been made simple by aggregation on the ProductID, ProductName and application of SUM() function to make total sales data come out. The users need to query the view which automatically will return summarized data without requiring a user-manual aggregation logic. With this method, it simplifies how formatted consistent and aggregated data could be presented directly towards users.

15.How would you allow and deny view permissions to a view to specific users?
You use the GRANT and REVOKE SQL statements to give and take away permissions on a view to selected users. This allows you to control access and modify what is displayed by a view.
Granting Permissions
You can grant individual rights such as SELECT, INSERT, UPDATE or DELETE to a view.
The syntax for granting select ability on the view SalesSummary for a user called user1 would be like this: GRANT SELECT ON SalesSummary TO user1;
If you want to give many different rights, say like above, you might have multiple GRANT commands.
GRANT SELECT, UPDATE ON SalesSummary TO user1;
Revoking Permissions:
The REVOKE command will be used to take away the permission from a user account.
The following statement removes the SELECT permission from the user1 on the SalesSummary view:
REVOKE SELECT ON SalesSummary FROM user1;
You can also do multiple deletions in the same command:
REVOKE SELECT, UPDATE ON SalesSummary FROM user1;
•Granting Permissions: The GRANT command allows the users to perform certain operations on the view, such as viewing or updating the data.
•Revoking Permissions: The REVOKE command deletes those permissions, so that users cannot access or change data according to your permissions.
These commands control the access to views and keep sensitive data safe so that only authorized users can access it.

16.How do you create indexed views to improve query performance?
You have to start by creating a view with the SCHEMABINDING option in order to create indexed views and improve query performance. This makes it so that the underlying tables cannot be modified as long as the view is in existence, otherwise an index cannot be created.
Here is how you might do this:
CREATE VIEW SalesSummary
WITH SCHEMABINDING AS
SELECT ProductID, SUM(SaleAmount) AS TotalSales
FROM dbo.Sales
GROUP BY ProductID;
You then create an unique clustered index on view. The clustered index places the result set of a view in physical storage hence data retrieval is faster when it does not have to recalculate the result set time and again for every query.
Using the following SQL statement
CREATE UNIQUE CLUSTERED INDEX IX_SalesSummary_ProductID ON SalesSummary(ProductID);
The SCHEMABINDING option binds a view to the schema of the underlying tables so they cannot be changed in such a way that would affect a view. The clustered index physically organizes data into a sequence of values placed on a disk or hard drive, which makes that data faster to access, and queries that need long complex calculations or aggregations will run much faster when optimized with indexed views.

17.How do you handle materialized views for performance optimization?
In an interview, handling materialized views for performance optimization usually comes up as improving query performance by storing the result of complex queries physically. Materialized views are not like regular views because they store data instead of calculating it on the fly, which makes them very beneficial for performance, especially when dealing with large datasets.
Refresh strategy to be reconsidered while building materialized views. There are a variety of refresh strategies for materialized views.
These include
1.Immediate Refresh: The view is populated as soon as it is created.
2.Complete Refresh: Data is recalculated completely every time it is refreshed, which would be very resource-intensive
3.Incremental (Fast) Refresh: only changed data gets refreshed which is the most efficient means of refreshing when the volume of data is large
4.On-Commit Refresh: Automatically refreshed when changes are made on the underlying tables.
Indexing on materialized views needs to be used along with filtering or joining to make query performance optimize. Also, partitioning the view helps in handling large amounts of data by splitting up huge chunks of data into smaller bits and hence it becomes more efficient.
Thus, you can administer these materialized views and obtain greatly improved performance on complex queries, diminish loading on underlying tables and, thus enhance the overall efficiency of a system at hand.

18.How do you make your view consistent and non-changing even with WITH CASCADE?
It allows WITH CASCADE to maintain view consistency and integrity, which ultimately implies that it maintains data integrity in the related tables. It is often used by means of foreign key constraints and automatically propagates changes created in the parent table over the child tables. For views, it ensures consistency as it reflects updates and deletes within the underlying tables and then also within the views. For example, consider the following view, which is joined with Orders and Customers with a foreign key relationship deleting a customer from the Customers table will cascade the change to the Orders table and the view, with no orphaned records:. WITH CASCADE ensures that all changes done in the parent table are updated automatically in all child tables and views related to it so that there is no inconsistency of data and referential integrity is maintained throughout the database.

19.How do you treat NULL values in views while keeping the data quality intact?

Treatment of NULL values in views is important while dealing with calculations or aggregation processes so that the data integrity can be maintained. Some common practices include using the COALESCE() function or NVL() function, if you are an Oracle user, so NULLs will not affect data integrity because of replacement with a default value. For example, consider the sales report view where some of the transactions have missing amounts. You can replace NULL with zero so that the report would show complete data.
Following is the SQL example.
CREATE VIEW SalesReport AS
SELECT
ProductID,
COALESCE(SUM(SaleAmount), 0) AS TotalSales
FROM Sales
GROUP BY ProductID;
In this expression, COALESCE(SUM(SaleAmount), 0) ensures that NULL values in the SaleAmount column are replaced with a value of 0 so that no missing data impacts the sales total. Thus, the output of the report is perfectly accurate even though some sale amounts are missing.

20.How do you use views to enforce business rules (e.g. discount eligibility)?
Using views to enforce business rules, like discount eligibility, involves adding logic directly into the view definition. This way you can ensure that only valid data based on the business rules is presented to the user, which simplifies application logic and ensures consistency. For instance, a business rule may be that a customer gets a discount only if his or her total purchase is more than 500. In a view, you could use a CASE statement in order to implement this kind of logic and return just those customers who qualify for a discount.
Here is an example in SQL
CREATE VIEW DiscountEligibleCustomers AS
SELECT
CustomerID,
TotalPurchase,
CASE
WHEN TotalPurchase > 500 THEN 'Eligible'
ELSE 'Not Eligible'
In this case, when checking whether a customer's purchases have totaled over 500 or not, it gives you 'Discount Eligible' or 'Not Eligible', enforcing the business rule immediately at the database level to always get the correct status at query time without you handling the logic at an application level. This avoids things being inconsistent, produces less errors and keeps enforcement centralised.