Introduction: Become a SQL Queries Superhero

When was the last time you thought about where you got the info behind your banking app showing your balance in a nanosecond? Or how about when Netflix suggested the movie JUST for you? The journey towards these outcomes starts with data and what you do with that data starts with SQL queries. Every superhero has to start with a foundational power, for everybody who codes, that foundational power starts with mastering SQL. 

Today, we humans engage with a mountain of apps, websites, and software but most of it has a layer of invisibility beneath it called data. If we want to contribute and consume in this ecosystem we are going to need to learn how we communicate with that data, get what we want, and do it all in an efficient manner. That is why learning SQL will go from being a technical skill to a literacy skill. So learning how to communicate with these systems using SQL queries will go from being an optional technical skill to a mandatory literacy skill. 

This blog post will guide you through the beginner's journey from zero to querying hero all the way to the level where you will be proud to say you can use SQL queries.

What Are SQL Queries and Why Should You Care?

SQL (Structured Query Language) is the standard language to manage and manipulate databases that are relational in nature. Generally speaking, a database is an organized collection of information, similar to a digital filing cabinet. A sql query is simply the command you can use to communicate with this cabinet in order to ask the cabinet to find information, add information, update information, or delete information.

Why should you care? Because the data is everywhere! Any time you search for something on Amazon - your SQL request is retrieving information from a database behind the scenes. When you check your bank balance, a specialized SQL request is pulling your account information for you. Without an efficient process to interact with a structured data set, you wouldn't be able to do either of these things. Learning how to build effective database queries will give you the opportunity to pull out and analyze, or manage that information that is so valuable. This will make you extremely valuable in almost any technology associated career.

Key Commands of SQL – Your Query Toolkit

To become proficient in writing database instructions, you need to understand the essential commands of SQL that form your query toolkit. These commands allow you to perform all the fundamental operations on a database.

SELECT: This is arguably the most common SQL command. It’s used to retrieve data from a database.

• Syntax: SELECT column1, column2 FROM table_name;
• Example: SELECT product_name, price FROM products; (This would get the names and prices of all products.)
• Use Case:Displaying a list of items on an e-commerce website.

INSERT: Use INSERT to add new rows of data into a table.

• Syntax: INSERT INTO table_name (column1, column2) VALUES (value1, value2);
• Example: INSERT INTO customers (first_name, last_name) VALUES ('Jane', 'Doe'); (Adds a new customer.)
• Use Case: Registering a new user on a website.

UPDATE: This command modifies existing records in a table.

• Syntax: UPDATE table_name SET column1 = new_value WHERE condition;
• Example: UPDATE products SET price = 29.99 WHERE product_name = 'Laptop Bag'; (Changes the price of a specific product.)
• Use Case: Changing a user's password or updating product stock levels.

DELETE: Use DELETE to remove existing records from a table.

• Syntax: DELETE FROM table_name WHERE condition;
• Example: DELETE FROM orders WHERE order_id = 101; (Removes an order.)
• Use Case: Deleting an outdated record or removing a user account.

Beyond these, there are crucial clauses and commands that refine your database interactions:

1. WHERE: Filters records based on a specified condition. It's often used with SELECT, UPDATE, and DELETE.

• Example: SELECT * FROM employees WHERE department = 'Sales'; (Gets all employees in the Sales department.)

2. ORDER BY: Sorts the result set of a SQL command in ascending or descending order.

• Example: SELECT product_name, price FROM products ORDER BY price DESC; (Lists products by price, highest first.)

3. GROUP BY: Groups rows that have the same values in specified columns into summary rows, often used with aggregate functions (like COUNT, AVG, SUM).

• Example: SELECT department, COUNT(employee_id) FROM employees GROUP BY department; (Counts employees per department.)

4. JOIN: Combines rows from two or more tables based on a related column between them. This is vital when data is spread across multiple tables.

• Example: SELECT orders.order_id, customers.first_name FROM orders JOIN customers ON orders.customer_id = customers.customer_id; (Links orders to customer names.)

Understanding these fundamental commands of SQL is paramount to interacting effectively with databases.

Level Up – Queries and Subqueries in SQL

Once you have a good understanding of the basic SQL commands, you can move on to more complex ones involving more complicated queries and subqueries in SQL. What is a subquery? A subquery (also called inner or nested query) is simply a SQL statement that is included in another SQL statement. 

Subqueries are powerful because you can perform actions that rely on the results from another query - thus making your requests to your database dynamic and more efficient. For instance, let's say you want to find all the products that are a higher price than the average price of all products. 

First, you would need to find the average price (one query), and then use that answer to find the products (another query). A subquery is going to enable you to do this all in one statement and much more elegantly:

• SQL
• SELECT product_name, price
• FROM products
• WHERE price > (SELECT AVG(price) FROM products);

Here, (SELECT AVG(price) FROM products) is the subquery. It runs first, returns the average price, and then the outer SQL command uses that value to filter the products.

When to use subqueries:

• To filter results based on an aggregate function.

• To check for the existence of records (EXISTS, NOT EXISTS).

• To retrieve values for use in IN, NOT IN, ANY, or ALL clauses.

Common mistakes to avoid:

• Returning multiple columns: A subquery used in a WHERE clause condition (like price > ...) must return only one column.

• Returning multiple rows where one is expected: If a subquery is used in a context expecting a single value (e.g., in a SELECT list or as a single comparison operand), ensure it returns only one row.

• Performance issues: For very large datasets, highly nested subqueries can sometimes be less performant than JOIN operations. Always consider alternatives and test performance.

Mastering subqueries unlocks a new level of complexity and power in your database skills, allowing you to tackle more intricate data retrieval and manipulation tasks.

Practice Makes Perfect – Beginner Challenges

The best way to solidify your understanding of SQL is through hands-on practice. Here are a few quick exercises to get you started on writing your own SQL queries:

1. Challenge 1: Select all employees from a table named Employees who work in the 'IT' department.

• Hint: Use SELECT and WHERE.

2. Challenge 2: Add a new product to a Products table with a product_name of 'Wireless Mouse' and a price of 25.00.

• Hint: Use INSERT INTO.

3. Challenge 3: Find the names of all customers who have placed an order, using JOIN between a Customers and an Orders table.

• Hint: You'll need SELECT and JOIN.

4. Challenge 4: Update the price of the 'Wireless Mouse' to 29.99.

• Hint: Use UPDATE and WHERE.

5. Challenge 5: Find the total number of orders placed by each customer.

• Hint: You'll need SELECT, COUNT(), and GROUP BY.

There are many free SQL practice tools and playgrounds available online where you can run these queries and experiment, such as W3Schools SQL TryIt Editor, SQL Fiddle, or DB-Fiddle. Don't be afraid to make mistakes; that's how you learn!

Learn CS&IT with SERI – From Queries to Careers

Knowing SQL is a great start but it is only one aspect of the vast, interesting and exciting world of Computer Science and Information Technology. If you are excited about delving deeper into your learning and working towards a future in technology, SERI’s CS & IT programs will teach you SQL and so much more! 

Our curriculum is designed to align with industry standards and includes practical database projects; this gives students the opportunity to have hands-on and practical training on complex SQL statements and managing databases in practice. 

With access to up-to-the-minute technologies, state-of-the-art laboratories and real faculty experience in the industry! SERI gives you the great support that you need to excel. SERI’s curriculum will prepare students with advanced applications in database concepts, design, administration and integrate knowledge in technology. SERI is the right program for your future!

Conclusion: Every Superhero Starts with a Single Query

Every superhero narrative starts with a first, critical step. For those just starting out as coders or as individuals who solve problems, that first step is frequently comfortable with SQL queries. We have examined what SQL commands are, we looked at the main commands of SQL (SELECT, INSERT, UPDATE, DELETE), we explored the query and subquery capabilities of SQL. The important thing to remember is to start small, to build a foundation, and constantly practice and explore. 

In order to clarify, SQL is not just for the specialized database administrator, or the specialized software developer, things change, systems change, skills change; SQL is for anyone who wants to understand and engage with the significant datasets that fuel so much of the world today. So practice your commands, tinker and experiment with how to put different structures together and remember, SQL isn’t just for coders; SQL is also for problem solvers. 

[Learn more about how SERI can help you launch your tech career and master SQL at learn CS&IT with SERI.]