SQL Basics – Learn Core SQL Concepts

What is SQL and Why Does It Matter?

SQL (Structured Query Language) is the universal language for interacting with relational databases. Whether you’re analyzing marketing performance, managing user data, or debugging backend systems—SQL is the tool of choice for querying, filtering, joining, and aggregating data.

How SQL Works

At its core, SQL lets you ask questions about data stored in tables. A table is like a spreadsheet: rows are individual records, and columns are fields like name, email, or signup_date.

The Building Blocks of SQL

• SELECT: The most basic command. It pulls data from a table.
  SELECT * FROM users;
• WHERE: Adds filters so you only get the rows you care about.
  SELECT * FROM users WHERE country = 'USA';
• ORDER BY: Sorts results.
  SELECT * FROM users ORDER BY signup_date DESC;
• LIMIT: Restricts the number of results.
  SELECT * FROM users LIMIT 10;

Understanding Joins

Often, data is spread across multiple tables. JOINS let you combine data based on shared fields.

• INNER JOIN: Only returns matches in both tables.
• LEFT JOIN: Returns all rows from the left table, even if there’s no match on the right.

Example:

pgsql

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SELECT users.name, orders.total  
FROM users  
INNER JOIN orders ON users.id = orders.user_id;


Common Mistakes for Beginners

• Using = instead of IN for multiple values
• Forgetting to alias tables in joins
• Mixing up WHERE and HAVING (WHERE filters rows before aggregation, HAVING filters after)

Next Steps

Practice writing queries using real data. Learn to GROUP, COUNT, and work with functions like AVG() and MAX(). As you get more confident, explore subqueries, views, and indexes.