What is a database?

Why not just a file or an array?

An array in your code lives in memory (RAM). When the server restarts, it is gone. A plain file survives restarts, but if two users save at the same moment the file can break, and finding one order among ten lakh means reading the whole file.

A database is a program built only for storing data. It keeps data safe on disk, lets thousands of users read and write at the same time without breaking anything, and finds rows quickly even in huge tables.

Tables, rows and columns

• A table holds one kind of thing: students, orders or trains.
• A column is one detail that every row has, like name or fare. Each column has a data type.
• A row is one item: one student, one order, one train.
• A primary key is a column whose value is unique for every row, like a roll number or a PNR. It lets you point to exactly one row.

SQL: the language of databases

SQL (Structured Query Language, said "S-Q-L" or "sequel") is the language you use to talk to a relational database. A relational database stores data in tables that can be linked to each other. CREATE TABLE makes a new table, INSERT adds rows and SELECT reads them.

CREATE TABLE trains (
  number INTEGER PRIMARY KEY,
  name TEXT NOT NULL,
  from_city TEXT,
  to_city TEXT,
  fare INTEGER
);

INSERT INTO trains (number, name, from_city, to_city, fare)
VALUES (12951, 'Mumbai Rajdhani', 'Mumbai', 'New Delhi', 3100);

SELECT * FROM trains;

SQL vs NoSQL in one paragraph: SQL databases (PostgreSQL, MySQL, SQLite) store data in tables with fixed columns and are the safe default for most apps, especially anything with money. NoSQL databases (like MongoDB) store flexible documents, similar to JSON, and suit data whose shape changes often. You will meet NoSQL in a later module.

Popular choices: PostgreSQL (powerful, free, the most common pick for new backends), MySQL (very widely used, for example by WordPress), and SQLite (a whole database in one file, used inside phones and for learning). The tasks on this site run SQLite inside your browser.