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Hello, Data! Meet SQL

Hello, Data! Meet SQL

Learn the fundamentals of SQL to efficiently query and filter data stored in relational databases.

Beyond files and APIs, data is often stored in relational databases—systems designed for efficient, structured storage and retrieval. Relational databases store data in tables, like a spreadsheet, with rows and columns. But unlike a single file, databases can connect multiple tables, enforce data consistency, and efficiently handle huge volumes of records.

To interact with and fetch data from these databases, we use Structured Query Language (SQL), the primary tool for interacting with relational databases. At its core, SQL lets us select the information we need, specify where it comes from, and narrow down results to exactly what’s relevant.

In this lesson, we’ll explore the fundamental building blocks of SQL queries—how to select data, apply filters, and return only what’s relevant.

Sample data: Employees table

Let’s start with a simple example table called employees to understand how SQL works. It stores information about employees in a company, including their ID, name, department, hire date, and salary.

EmployeeID

Name

Department

HireDate

Salary

1

Alice Johnson

Engineering

2020-03-15

85,000.00

2

Bob Smith

Marketing

2019-07-22

65,000.00

3

Carol Martinez

Sales

2021-01-08

72,000.00

4

David Liu

Engineering

2018-11-02

95,000.00

5

Eva Gómez

HR

2022-05-16

58,000.00

6

Frank O’Connor

Finance

2017-09-30

78,000.00

7

Grace Patel

Engineering

2023-02-12

80,000.00

8

Hiro Tanaka

Support

2020-12-01

60,000.00

9

Isabella Rossi

Marketing

2021-06-25

67,000.00

10

Arya Stark

Sales

2019-04-10

71,000.00

Selecting data: The SELECT and FROM clauses

Two SQL clauses, SELECT and FROM, are essential when querying a database. They work together to tell the database what data we want and where to find it.

SELECT is how we specify the exact columns—or pieces of information—we want to see in our results. For example, if we only care about employee names and salaries, we tell SQL to select only those columns.

But data doesn’t float in space—it lives inside tables. That’s where FROM comes in. It tells SQL which table to pull the data from. As databases often contain many tables, we must specify the source.

Together, SELECT and FROM answer two key questions:

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