What Is SQL and Databases? A Simple Guide for Students

A database is an organised store of data that many people and programs can read and change safely at the same time. Your college's marks, a bank's accounts and every order in an online shop live in databases.

SQL (Structured Query Language) is the language used to ask a relational database questions, such as "show me every student with attendance below 75 percent". It is one of the most useful skills in IT, because developers, testers, analysts and data scientists all use it.

Tables, rows and keys

A relational database keeps data in tables, like spreadsheets with strict rules. A students table might have columns for roll number, name and department; each row is one student.

A primary key is a column that uniquely identifies each row, such as the roll number. A foreign key links tables: a marks table stores the student's roll number to say whose marks they are. Splitting data like this avoids writing the same details again and again.

The queries you must know

• SELECT ... FROM ... WHERE: read rows that match a condition.
• ORDER BY and LIMIT: sort and take the top results.
• GROUP BY with COUNT, SUM, AVG: totals per department, per month, per product.
• JOIN: combine tables, such as students with their marks.
• INSERT, UPDATE, DELETE: change data (always write the WHERE first on UPDATE and DELETE).

SQL and NoSQL

Relational databases such as MySQL, PostgreSQL and SQL Server are the default for most business data. NoSQL databases such as MongoDB or Firestore store flexible documents and suit some apps well. Learn SQL first: the ideas of keys, indexes and relationships help you with every database.

How to practise

Before interviews, read SQL interview questions for freshers. The Database Developer track and the PostgreSQL database internship take you further.

• Install PostgreSQL or MySQL, or use an online SQL playground.
• Create a small college database: students, courses, enrolments, marks.
• Write twenty questions in plain English, then answer each with a query.
• Learn what an index is and see how it speeds up a search on a big table.