How Every Beginner Should Know How to Write SQL Queries?

Writing SQL queries is the core skill every backend, full-stack, or data engineer must learn. SQL Server may look complicated initially, but once you understand a few foundational queries like:

• SELECT

• WHERE

• ORDER BY

• JOIN

• GROUP BY

• Aggregations (COUNT, SUM, AVG)

you can query almost any business database.

This article takes a practical approach. The examples are based on a real-world automotive service management system, similar to the database from Article 1. Every query comes with:

• The problem scenario

• How the query works

• A working SQL script example

This is written for beginners but structured in a way that builds real production readiness.

Real-World Context

Our database has three tables:

Customer

CREATE TABLE Customer (
    CustomerId INT PRIMARY KEY,
    FullName NVARCHAR(100),
    City NVARCHAR(100)
);

Car

CREATE TABLE Car (
    CarId INT PRIMARY KEY,
    CustomerId INT,
    Brand NVARCHAR(50),
    Model NVARCHAR(50),
    FOREIGN KEY (CustomerId) REFERENCES Customer(CustomerId)
);

ServiceRecord

CREATE TABLE ServiceRecord (
    ServiceId INT PRIMARY KEY,
    CarId INT,
    ServiceDate DATE,
    Cost DECIMAL(10,2),
    FOREIGN KEY (CarId) REFERENCES Car(CarId)
);

We will write SQL queries based on this data model.

SELECT: Retrieving Data

The SELECT command is used to fetch data from a table.

Example: Get all customers.

SELECT * FROM Customer;

While SELECT * is useful during development, in production always use explicit columns.

SELECT CustomerId, FullName, City FROM Customer;

WHERE: Filtering Records

Example: Customers from Mumbai.

SELECT FullName, City
FROM Customer
WHERE City = 'Mumbai';

You can use comparison operators:

• =

• !=

• >

• <

• BETWEEN

• LIKE

Example: Find customers whose name starts with "P".

SELECT FullName
FROM Customer
WHERE FullName LIKE 'P%';

ORDER BY: Sorting Results

Sort customer list alphabetically.

SELECT FullName, City
FROM Customer
ORDER BY FullName ASC;

Sort by most recent service first:

SELECT CarId, ServiceDate, Cost
FROM ServiceRecord
ORDER BY ServiceDate DESC;

JOIN: Combining Data from Multiple Tables

JOINs allow you to read related data across tables.

INNER JOIN

Get all cars with customer names.

SELECT
    Customer.FullName,
    Car.Brand,
    Car.Model
FROM Car
INNER JOIN Customer
    ON Customer.CustomerId = Car.CustomerId;

LEFT JOIN

Get all customers even if they do not own a car.

SELECT
    Customer.FullName,
    Car.Brand,
    Car.Model
FROM Customer
LEFT JOIN Car
    ON Customer.CustomerId = Car.CustomerId;

RIGHT JOIN (Rarely Used)

SELECT *
FROM Customer
RIGHT JOIN Car
    ON Customer.CustomerId = Car.CustomerId;

GROUP BY: Summarizing Data

GROUP BY is used with aggregate functions like COUNT, SUM, AVG.

Example: Count how many cars each customer owns.

SELECT
    Customer.FullName,
    COUNT(Car.CarId) AS TotalCars
FROM Customer
LEFT JOIN Car
    ON Customer.CustomerId = Car.CustomerId
GROUP BY Customer.FullName;

Example: Total service cost per car.

SELECT
    Car.Brand,
    Car.Model,
    SUM(ServiceRecord.Cost) AS TotalServiceCost
FROM Car
INNER JOIN ServiceRecord
    ON Car.CarId = ServiceRecord.CarId
GROUP BY Car.Brand, Car.Model;

HAVING: Filtering Group Results

Unlike WHERE, HAVING works after grouping.

Example: Customers who own more than one car.

SELECT
    Customer.FullName,
    COUNT(Car.CarId) AS CarCount
FROM Customer
LEFT JOIN Car
    ON Customer.CustomerId = Car.CustomerId
GROUP BY Customer.FullName
HAVING COUNT(Car.CarId) > 1;

TOP: Limit Results

Example: Get the most expensive service.

SELECT TOP 1 *
FROM ServiceRecord
ORDER BY Cost DESC;

DISTINCT: Remove Duplicates

Example: List all unique cities.

SELECT DISTINCT City
FROM Customer;

Combining Multiple Clauses

Example:
Get top 5 customers with the highest total service spending.

SELECT TOP 5
    Customer.FullName,
    SUM(ServiceRecord.Cost) AS TotalSpent
FROM Customer
JOIN Car ON Customer.CustomerId = Car.CustomerId
JOIN ServiceRecord ON Car.CarId = ServiceRecord.CarId
GROUP BY Customer.FullName
ORDER BY TotalSpent DESC;

This is similar to a real dashboard/report query.

Real-World Case Study

A service company initially exported all service records to Excel and manually calculated revenue. It took nearly 5 hours per week and had frequent mistakes.

After learning GROUP BY queries:

• Revenue reports were generated instantly.

• Customer trends became visible.

• The business started offering targeted service packages.

SQL skills directly improved business outcomes.

Common Beginner Mistakes

Mistake	Better Practice
Using SELECT * in production	Use explicit column names
Forgetting WHERE condition	May return huge datasets
Using RIGHT JOIN instead of LEFT JOIN	LEFT JOIN is usually easier and logical
Using HAVING for non-aggregates	Use WHERE instead
No ORDER BY in reports	Reports appear random

Best Practices

• Always format your SQL for readability.

• Use meaningful aliases.

• Use LIMIT/TOP when testing.

• Follow naming conventions.

Example with formatting

SELECT
    c.FullName,
    SUM(sr.Cost) AS TotalSpent
FROM Customer c
JOIN Car ca ON c.CustomerId = ca.CustomerId
JOIN ServiceRecord sr ON ca.CarId = sr.CarId
GROUP BY c.FullName
ORDER BY TotalSpent DESC;

Summary

SQL querying begins with understanding and practicing:

• SELECT

• JOIN

• GROUP BY

• Filtering and sorting

These building blocks enable beginners to confidently query real business systems and prepare for more advanced topics like indexing, triggers, optimization, and stored procedures.

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