Understanding MSSQL Subqueries and Views: A Beginners Guide – Part 6

A young explorer venturing into the world of MSSQL Subqueries and Views


Hello, explorers! Welcome to our exciting journey into the fascinating world of databases. Today, our adventure will take us deep into the heart of MSSQL, which stands for Microsoft SQL Server. It’s a powerful tool used by many businesses and organizations to store and manage their data.

Now, you might be wondering, “What are databases?” Well, think of a database like a big, digital treasure chest that stores all kinds of information – from the scores of your favorite video games to the number of steps you’ve walked in a day. And MSSQL is like the key that helps us unlock this treasure chest!

In our journey today, we’ll be focusing on two important keys – Subqueries and Views. These might seem like big, complicated words, but don’t worry! Just like how every great adventurer starts with small steps, we’ll break down these concepts into fun, bite-sized pieces.

Subqueries are like the clues in a treasure hunt. They help us find the exact piece of information we’re looking for in our big treasure chest. And Views? They’re like maps that guide us directly to the most valuable treasures, saving us time and effort.

By the end of this blog, not only will you understand what these words mean, but you’ll also be using them to unlock your own treasure chests of data! So, buckle up, young explorers, as we set sail on this exciting adventure into the world of MSSQL Subqueries and Views. Let’s go! 😊

Deep Understanding

What are Subqueries?

Imagine you’re playing a game of hide and seek. You know your friend is hiding somewhere in the house, but you don’t know exactly where. So, what do you do? You start by checking each room (this is your main query). But once you’re in a room, you need to look behind the furniture, under the bed, or inside the closet (these are your subqueries).

In the world of MSSQL, a subquery works in a similar way. It’s essentially a query that’s nested inside another query. A subquery can return data that will be used in the main query as a condition to further restrict the data to be retrieved. Subqueries can be used in different ways and at different locations inside a query: Here is an example:

In this example, the subquery calculates the average salary of all employees. The outer query then retrieves the employees who earn more than the average salary.

What are Views?

Now, let’s think about your favorite superhero movie. You don’t have to watch the whole movie to remember the best scenes, right? You can just view those scenes directly.

In MSSQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. Here is an example:

CREATE VIEW [Fast Food Orders] AS
SELECT CustomerName, OrderDate
FROM Customers
JOIN Orders
ON Customers.CustomerID = Orders.CustomerID
WHERE Category = 'Fast Food';

In this example, [Fast Food Orders] is a view that stores the customer name and order date of all fast food orders. This view is generated from the Customers and Orders tables. The advantage of a view is that it allows you to see specific data located in multiple tables as if it was a single table, without changing the schema of the original tables.

Performing Operations with Practical Scenarios

Imagine we’re on a virtual safari in a zoo. The zoo has a database that keeps track of all the animals. Each animal has a type (like ‘Mammal’, ‘Bird’, ‘Reptile’, etc.), a name, an age, and the continent they’re originally from.

Using Subqueries

Let’s say we want to find out the names of all the mammals in the zoo that are older than the average age of mammals. We can use a subquery to do this:

SELECT Name FROM Animals
WHERE Type = 'Mammal' AND Age > (SELECT AVG(Age)

FROM Animals WHERE Type = 'Mammal');

In this query, the subquery (SELECT AVG(Age) FROM Animals WHERE Type = 'Mammal') calculates the average age of all mammals in the zoo. The main query then uses this information to find all mammals that are older than this average age.

Using Views

Now, suppose we frequently need to know the names and ages of all the birds in the zoo. Instead of writing the same query every time, we can create a view:

SELECT Name, Age FROM Animals WHERE Type = 'Bird';

Now, whenever we want to see this information, we can simply use:


This will show us the names and ages of all the birds, just like we had a special ‘Birds’ table!

Remember, these are just examples. The actual queries you’ll write will depend on how your database is structured and what information you’re looking for. But with these tools in your toolbox, you’ll be well-equipped to go on your own data adventures!

Best Practices

Keeping Subqueries Simple

Subqueries are a powerful tool in SQL, allowing you to perform operations that would otherwise be difficult or impossible. However, with great power comes great responsibility. Complex subqueries can make your database slow and inefficient, especially if they involve large amounts of data or multiple levels of nesting.

When to use it: Use subqueries when you need to perform an operation that can’t be done with a regular query. For example, if you need to compare each record in your table to an aggregate value (like the average or maximum), a subquery is a perfect tool for the job.

When to avoid it: Avoid using subqueries when a simpler alternative exists. If the same operation can be performed with a JOIN or a window function, it’s usually a better choice. Also, avoid unnecessary nesting of subqueries, as it can make your queries harder to understand and slower to run.

Using Views for Frequently Accessed Data

Views are like shortcuts to your data. They allow you to save a query and then refer to it like it’s a table. This can make your work faster and easier, especially if you find yourself running the same complex query over and over again.

When to use it: Use views when you have a complex query that you run frequently. Instead of typing out the whole query each time, you can create a view and then SELECT from that view just like it’s a regular table. This can save you a lot of time and make your code cleaner and easier to read.

When to avoid it: Avoid using views as a way to circumvent good database design. If you find yourself creating a lot of views, it might be a sign that your tables aren’t structured well. Remember, views are a tool to make your work easier, not a crutch to lean on when your database design is lacking.

Remember, these are just guidelines. Every database and every situation is unique, so what works best in one case might not work as well in another. Always consider the specific needs and constraints of your project when deciding how to structure your queries.

Additional Examples from Actual Industry

In the bustling world of business, data is king. Companies of all sizes use databases to store and manage their data, and SQL is one of the most popular tools for interacting with these databases. Subqueries and views are two powerful features of SQL that businesses use every day. Let’s look at a few examples:

Online Store

Imagine an online store that sells a variety of products. They have a database with tables for CustomersOrders, and Products.

Subquery Example: The store wants to find all customers who have made more than ten purchases. They could use a subquery to achieve this:

SELECT CustomerID, CustomerName FROM Customers
WHERE CustomerID IN (
SELECT CustomerID FROM Orders

In this query, the subquery (SELECT CustomerID FROM Orders GROUP BY CustomerID HAVING COUNT(OrderID) > 10) finds the IDs of all customers who have made more than ten orders. The main query then uses this list to retrieve the names of these customers.

View Example: The store frequently needs to know the total sales for each product. Instead of calculating this every time, they could create a view:

SELECT ProductID, ProductName, SUM(Quantity) as TotalSales FROM Orders
GROUP BY ProductID, ProductName;

Now, whenever they want to see the total sales for each product, they can simply use SELECT * FROM ProductSales;.

Healthcare Provider

Consider a healthcare provider with a database containing tables for PatientsDoctorsAppointments, and Treatments.

Subquery Example: The healthcare provider wants to find all doctors who have treated more than 100 patients. They could use a subquery for this:

SELECT DoctorID, DoctorName FROM Doctors
SELECT DoctorID FROM Treatments

View Example: The healthcare provider frequently needs to access the treatment history of patients. They could create a view for this:

CREATE VIEW PatientHistory AS
SELECT PatientID, PatientName, DoctorName, TreatmentDate, TreatmentType

FROM Patients
JOIN Treatments ON Patients.PatientID = Treatments.PatientID
JOIN Doctors ON Treatments.DoctorID = Doctors.DoctorID;

These are just a few examples of how businesses use subqueries and views in the real world. The possibilities are endless and only limited by the complexity of the data and the creativity of the database users. 


And there you have it, young explorers! You’ve embarked on a thrilling journey into the world of MSSQL and emerged with newfound knowledge about subqueries and views. These are not just abstract concepts, but powerful tools that you can use to unlock the secrets hidden in your data.

Remember, every big concept starts with small steps. Today, you’ve taken a significant step towards becoming a database pro. But don’t stop here. There’s a whole world of data out there waiting to be discovered!

Keep exploring, keep questioning, keep learning. Each query you write, each result you find, brings you one step closer to mastering MSSQL. And as you continue on this path, you’ll find that these concepts become second nature to you.

But more importantly, you’ll realize that learning is not just about understanding new concepts. It’s about seeing the world from a different perspective. It’s about realizing that behind every piece of data, there’s a story waiting to be told.

So, keep that curiosity alive. Keep asking questions. Keep seeking answers. And who knows? One day, you might not just be using databases. You might be designing them!

And remember, in the world of data, you’re not just an explorer. You’re also a storyteller. And with tools like subqueries and views at your disposal, you’re well on your way to telling some truly amazing stories.

So, here’s to you, future database pros! May your queries always return the results you’re looking for. Happy exploring, and until next time, keep learning and keep growing! 😊

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