What is SQL Query

Introduction

Welcome to the world of SQL queries! A query is the fundamental way we communicate with databases to retrieve information.

In this tutorial, you will learn:

What a SQL query is and why we use it
The basic structure of a SQL query
How queries are processed by the database
The difference between queries and other SQL commands

What is a Query?

A SQL query is a request for data from a database. Think of it like asking a question - you send a query to the database, and it responds with the information you requested.

The Word "Query" Means "Question"

In everyday language, to query something means to ask about it. In SQL, a query is quite literally a question you ask the database:

Real-World Question	SQL Query Equivalent
"How many customers do we have?"	Count all rows in customers table
"Who are our customers in São Paulo?"	Find customers where city = 'sao paulo'
"What are our top-selling products?"	Find products with highest sales counts
"Which orders are still pending?"	Find orders where status = 'pending'

Why Do We Need Queries?

Databases store enormous amounts of data - millions or even billions of rows. Without queries, you would have to:

Look through every single record manually
Remember where specific information is stored
Perform calculations by hand

Queries let you:

Find specific data among millions of records in seconds
Filter to see only what matters
Sort data in meaningful ways
Calculate totals, averages, and other statistics
Combine information from multiple tables

The Anatomy of a SQL Query

Every SQL query follows a basic structure. The simplest possible query looks like this:

SELECT column_name FROM table_name;

Breaking Down the Parts

Part	Purpose	Required?
`SELECT`	Specifies which columns to retrieve	Yes
`column_name`	The actual column(s) you want	Yes
`FROM`	Indicates which table to query	Yes
`table_name`	The name of the table	Yes
`;`	Marks the end of the statement	Recommended

Example: A Simple Query

SELECT customer_city FROM olist_customers_dataset;

This query asks the database: "Give me the city column from the customers table."

Multiple Columns

You can request multiple columns by separating them with commas:

SELECT customer_city, customer_state FROM olist_customers_dataset;

All Columns

To get every column in the table, use the asterisk *:

SELECT * FROM olist_customers_dataset;

The asterisk is a wildcard meaning "everything."

Annotated SQL query showing each clause labeled with its purpose: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY

How Database Processes a Query

When you send a query to the database, it goes through several steps:

1. Parsing

The database reads your query and checks if it follows correct SQL syntax. If you have a typo or syntax error, it will reject the query.

2. Validation

The database verifies that:

The table you mentioned exists
The columns you requested exist in that table
You have permission to access this data

3. Optimization

The database creates an execution plan - the most efficient way to get your data. This is like planning the fastest route before a road trip.

4. Execution

The database follows its plan to retrieve the data, reading from storage and processing as needed.

5. Return Results

Finally, the database sends back the results - a table of data matching your query.

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│  Your SQL   │ ──▶ │  Database   │ ──▶ │   Results   │
│   Query     │     │  Processing │     │   (Data)    │
└─────────────┘     └─────────────┘     └─────────────┘

Queries vs Other SQL Commands

Not all SQL commands are queries. Understanding the difference is important:

Queries (DQL - Data Query Language)

Purpose: Read data from the database
Keyword: SELECT
Effect: Does NOT modify the database

SELECT * FROM customers;  -- This is a query

Non-Query Commands

These commands modify the database:

Type	Commands	Purpose
DML	INSERT, UPDATE, DELETE	Modify data
DDL	CREATE, ALTER, DROP	Modify structure
DCL	GRANT, REVOKE	Modify permissions

INSERT INTO customers ...;  -- NOT a query (adds data)
UPDATE customers SET ...;   -- NOT a query (changes data)
DELETE FROM customers ...;  -- NOT a query (removes data)

Why This Matters

Queries are safe - they only read data, never change it
In practice, 80% of SQL work involves queries
Learning queries first is the foundation for everything else

Practice

Let's write some simple queries to get comfortable with the basic syntax. Try running these queries against our Brazilian E-Commerce database.

Exercise 1: Your First Query

Run this query to see customer IDs and cities from our customers table:

-- Your first SQL query!
-- This retrieves customer_id and customer_city from the customers table

SELECT customer_id, customer_city
FROM olist_customers_dataset
LIMIT 10;

Exercise 2: Select All Columns

Use the asterisk to see all columns in the orders table:

-- Select ALL columns from the orders table
-- The * (asterisk) means "everything"

SELECT *
FROM olist_orders_dataset
LIMIT 5;

Exercise 3: Try It Yourself

Now write your own query! Select the product_id and price columns from the olist_order_items_dataset table.

-- Write a query to select product_id and price
-- from the olist_order_items_dataset table

SELECT product_id, price
FROM olist_order_items_dataset
LIMIT 10;

Common Query Mistakes

When writing queries, beginners often make these mistakes:

1. Misspelling Table or Column Names

-- Wrong: 'costumer' instead of 'customer'
SELECT * FROM olist_costumers_dataset;

-- Correct
SELECT * FROM olist_customers_dataset;

2. Forgetting FROM Clause

-- Wrong: No FROM clause
SELECT customer_city;

-- Correct
SELECT customer_city FROM olist_customers_dataset;

3. Using Wrong Quote Marks

-- Wrong: Double quotes for string values (in most databases)
SELECT * FROM customers WHERE city = "New York";

-- Correct: Single quotes for strings
SELECT * FROM customers WHERE city = 'New York';

4. Missing Commas Between Columns

-- Wrong: No comma between columns
SELECT customer_city customer_state FROM olist_customers_dataset;

-- Correct
SELECT customer_city, customer_state FROM olist_customers_dataset;

Summary

Congratulations! You now understand what SQL queries are and how they work.

Key Takeaways

✅ A query is a request for data from a database using the SELECT statement

✅ The basic query structure is: SELECT columns FROM table;

✅ Use * to select all columns

✅ Queries only read data - they never modify the database

✅ The database parses, validates, optimizes, and executes your query

Query Cheat Sheet

-- Select specific columns
SELECT column1, column2 FROM table_name;

-- Select all columns
SELECT * FROM table_name;

-- Limit results (good practice for large tables)
SELECT * FROM table_name LIMIT 10;

What's Next?

Now that you understand what queries are, let's learn about the components that make up more powerful SQL queries - including filtering with WHERE, sorting with ORDER BY, and grouping with GROUP BY.