What is SQL?

SQL (Structured Query Language) is the standard language used to create, read, update, and delete data stored in relational databases. Where a spreadsheet stores data in flat sheets, a relational database organizes it into linked tables, and SQL is how you ask questions of — and make changes to — that data.

Why SQL is used

Almost every application that stores structured data — from banking systems to e-commerce sites to internal admin tools — relies on a relational database underneath. SQL gives every one of them a common, declarative way to retrieve exactly the data needed, filter and combine it across tables, and keep it consistent, without each application inventing its own storage format.

Features of SQL

• Declarative querying — you describe what data you want, not how to fetch it
• Joins across multiple related tables
• Aggregation (COUNT, SUM, AVG, GROUP BY)
• Filtering and sorting (WHERE, ORDER BY)
• Data definition (CREATE, ALTER, DROP)
• Transactions for safe, all-or-nothing updates

Advantages of SQL

• Widely supported — nearly every relational database understands it
• Efficient at handling large, structured datasets
• Enforces data integrity through types, keys, and constraints
• Mature tooling, documentation, and community knowledge
• Readable, close to plain-English syntax

How SQL works with data

Data lives in tables made up of rows and columns. A statement like SELECT targets one or more tables, optionally filters rows with WHERE, and returns the matching data as a result set — the same shape you see in the query results below.

SQL syntax

SELECT column1, column2
FROM table_name
WHERE condition
ORDER BY column1;

Example: Basic SELECT

SELECT name, department, salary
FROM employees
WHERE salary > 60000
ORDER BY salary DESC;

Example: GROUP BY

SELECT department, COUNT(*) AS headcount, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
ORDER BY avg_salary DESC;

Ways to run SQL

1. Directly in a database client or admin tool (e.g. psql, MySQL Workbench)
2. Embedded in application code via a database driver or ORM
3. In-browser, as in the live editor on this page (via SQLite compiled to WebAssembly)

SQL dialects

Best practices

• Select only the columns you need, not SELECT *, in production code
• Filter early with WHERE rather than pulling everything and filtering in code
• Use meaningful table and column names
• Index columns you filter or join on frequently
• Wrap multi-step changes in a transaction