What is SQL? – Structured Query Language Explained

What is SQL?

SQL stands for Structured Query Language. It is the universal language used to create, read, update, and delete data stored in a relational database. Almost every application you have ever used — social networks, banking apps, e-commerce stores, SaaS platforms — stores its data in a relational database that is managed with SQL.

SQL is a declarative language: you describe what data you want, not how to retrieve it. You write SELECT * FROM customers WHERE country = 'BD' and the database engine figures out the most efficient way to find those rows. Compare this to imperative programming where you would loop over every record yourself.

A Brief History of SQL

SQL was invented in 1974 at IBM Research by Donald D. Chamberlin and Raymond F. Boyce. The original name was SEQUEL (Structured English Query Language), later shortened to SQL to avoid a trademark conflict. It was designed to work with IBM's System R — the world's first relational database prototype, built on Edgar F. Codd's 1970 relational model paper.

SQL has been standardised by both ANSI and ISO since 1986. The core language is stable and consistent across all major databases — which is why SQL skills you learn today are directly transferable between MySQL, PostgreSQL, SQLite, and SQL Server.

How SQL Works

When you run a SQL statement, it goes through several stages inside the database engine before any data is touched:

1. Parsing — The engine reads your SQL text and checks it for syntax errors, producing an abstract syntax tree (AST).
2. Semantic analysis — The engine verifies that the tables and columns you referenced actually exist and that you have permission to access them.
3. Query planning — The query planner generates one or more execution plans and estimates their cost (using statistics about table sizes and indexes).
4. Optimisation — The optimiser picks the lowest-cost plan — for example, deciding whether to use an index or scan the full table.
5. Execution — The storage engine executes the plan: reading pages from disk, applying filters, joining tables, sorting results.
6. Results — The final result set is sent back to your client application.

SQL Dialects

Every major database implements the ANSI SQL standard and then adds its own extensions. These variations are called dialects. The core syntax — SELECT, INSERT, UPDATE, DELETE, WHERE, JOIN — is identical across all of them. Differences appear in advanced features like string functions, date handling, and procedural extensions.

Your First SQL Query

The simplest SQL statement you can run is a SELECT with no table at all. Open any SQL client — SQLiteOnline.com, MySQL Workbench, psql, or the SQLite shell — and type:

SELECT 'Hello, SQL!';

Now let's look at a realistic example. Given a customers table, here are the four fundamental SQL operations:

-- SELECT: read data
SELECT name, email FROM customers WHERE country = 'BD';

-- INSERT: add a new row
INSERT INTO customers (name, email, country)
VALUES ('Rahim', 'rahim@example.com', 'BD');

-- UPDATE: modify existing data
UPDATE customers SET email = 'rahim.new@example.com' WHERE id = 42;

-- DELETE: remove a row
DELETE FROM customers WHERE id = 42;

Notice how readable these statements are. SELECT name, email FROM customers WHERE country = 'BD' reads almost like plain English: "give me the name and email from the customers table where the country is BD." This is SQL's declarative nature in action.

What SQL Can Do — The Four Command Groups

SQL commands are grouped into four categories based on their purpose: