SQL Basics: SELECT, FROM, and WHERE Statements

A beginner's guide to understanding the fundamental building blocks of SQL queries.

What is SQL?

SQL (Structured Query Language) is the standard language for interacting with relational databases. Whether you're retrieving data, updating records, or analyzing information, SQL is the tool that makes it possible. Think of SQL as a specialized language that allows you to "talk" to databases and tell them exactly what information you need.

At its core, SQL is designed to be readable and somewhat similar to English, making it more approachable than many programming languages. The basic structure of an SQL query includes several key components, with the most fundamental being the SELECT, FROM, and WHERE statements.

Basic SQL Query Structure

The most common SQL operation is querying data, which follows this basic pattern:

-- Basic SQL query structure
SELECT column1, column2, ...
FROM table_name
WHERE condition;

Let's break down each component:

•SELECT: Specifies which columns you want to retrieve
•FROM: Indicates which table(s) to query data from
•WHERE: Filters the results based on specified conditions

The SELECT Statement

The SELECT statement is the starting point of most SQL queries. It specifies which columns you want to retrieve from the database.

-- SELECT examples
-- Select specific columns
SELECT first_name, last_name, email FROM customers;

-- Select all columns
SELECT * FROM customers;

-- Select with column alias
SELECT first_name AS fname, last_name AS lname 
FROM customers;

Key Points About SELECT

•Use commas to separate multiple columns
•Use * as a wildcard to select all columns (use sparingly in real applications)
•Use AS keyword to create column aliases for more readable results
•Column order in the SELECT statement determines the order in the results

The FROM Statement

The FROM statement specifies which table or tables to query data from. This is where you tell the database where to find the columns you've requested in the SELECT statement.

-- FROM examples
-- Basic table selection
SELECT * FROM customers;

-- Using table alias
SELECT c.first_name, c.last_name 
FROM customers AS c;

-- Multiple tables
SELECT customers.name, orders.order_date 
FROM customers, orders 
WHERE customers.id = orders.customer_id;

Working with Tables

•Every query must have a FROM clause (except for simple calculations)
•Table aliases (using AS) make queries more readable, especially with multiple tables
•When querying multiple tables, you'll typically use joins (covered in more advanced articles)

The WHERE Statement

The WHERE statement filters your query results based on specific conditions. This is how you narrow down the data to just what you need.

-- WHERE examples
-- Simple comparison
SELECT * FROM products WHERE price > 50;

-- Text matching
SELECT * FROM customers WHERE city = 'New York';

-- Multiple conditions
SELECT * FROM orders 
WHERE order_date >= '2023-01-01' 
  AND total_amount < 100;

Filtering with WHERE

•The WHERE clause is optional, but highly recommended to limit results
•Conditions evaluate to TRUE, FALSE, or UNKNOWN (for NULL values)
•Only rows where the condition evaluates to TRUE are included in the result
•String values are typically case-sensitive and must be enclosed in quotes

Logical Operators

Combine multiple conditions using logical operators:

-- Logical operators
-- Using AND
SELECT * FROM employees 
WHERE department = 'Sales' 
  AND hire_date > '2022-01-01';

-- Using OR
SELECT * FROM products 
WHERE category = 'Electronics' 
  OR price > 100;

-- Using NOT
SELECT * FROM customers 
WHERE NOT country = 'USA';

Comparison Operators and Special Conditions

Use various operators to create precise filtering conditions:

-- Comparison operators
SELECT * FROM products WHERE price >= 50;  -- Greater than or equal
SELECT * FROM products WHERE price < 100;  -- Less than
SELECT * FROM products WHERE price <> 0;   -- Not equal to

-- BETWEEN operator
SELECT * FROM products 
WHERE price BETWEEN 10 AND 50;

-- IN operator
SELECT * FROM customers 
WHERE country IN ('USA', 'Canada', 'Mexico');

-- LIKE operator (pattern matching)
SELECT * FROM customers 
WHERE email LIKE '%gmail.com';  -- Ends with gmail.com

Understanding Data Types

When writing SQL queries, it's important to understand different data types:

Common Data Types

•TEXT: String values (enclosed in quotes)
•INTEGER: Whole numbers
•DECIMAL/NUMERIC: Numbers with decimals
•DATE/DATETIME: Date and time values
•BOOLEAN: True/false values

Working with Values

•Text values need single or double quotes: 'apple' or "apple"
•Numbers don't need quotes: 42, 3.14
•Dates typically use quotes and a specific format: '2023-05-25'
•Boolean values are typically TRUE or FALSE (some databases use 1/0)

SQL Best Practices for Beginners

Query Writing Tips

•Use clear, consistent formatting
•Be specific with column selection (avoid SELECT * in production)
•Use meaningful aliases for clarity
•Start with simple queries and build up complexity

Common Mistakes to Avoid

•Forgetting quotes around text values
•Missing commas between column names
•Using incorrect comparison operators
•Complex queries without proper testing

Practice Examples

Try these examples to practice basic SQL statements:

Finding All Products in a Category

SELECT name, price FROM products WHERE category = 'Electronics';

Customers from Specific Regions

SELECT first_name, last_name, city FROM customers WHERE country IN ('USA', 'Canada');

Recent Orders

SELECT order_id, customer_id, order_date FROM orders WHERE order_date >= '2023-01-01';

Next Steps

Now that you understand the basics of SQL queries, continue your learning journey:

Working with JOINs →

Learn how to combine data from multiple tables

Advanced Filtering →

Master complex WHERE conditions and filtering techniques