SQL

Why this matters

Spreadsheets are great until the data grows too big or lives inside a company system. Banks, telcos, mobile money operators, government data units and many remote employers keep their records in databases, and the language for asking those databases questions is SQL (often said "sequel" or "S Q L"). It appears regularly in Ghanaian analyst and BI adverts and in remote junior analyst roles. Learning it opens doors that spreadsheets alone cannot.

What a database is

A relational database stores data in tables, much like sheets in a spreadsheet. Each table has columns with fixed types, such as text, whole numbers or dates, and rows of records. Tables are linked by shared columns called keys. For example, an invented mobile money company might have a customers table and a transactions table, linked by customer_id.

The big differences from a spreadsheet:

• Databases can hold millions of rows without slowing down your phone or laptop.
• Many people can use the same data at once, safely.
• You do not edit cells by hand. You write a query that describes what you want, and the database returns the answer.

What SQL looks like

SQL reads almost like an English sentence:

SELECT district, amount
FROM transactions
WHERE amount > 500;

This asks for the district and amount of every transaction above 500. You will learn to filter, sort, combine tables and summarise them with counts and totals.

Analyst SQL is mostly reading

As an analyst you mostly read data with SELECT queries. You rarely create or delete tables; database administrators and engineers handle that. This makes SQL much less frightening: you cannot break anything by asking a question, and most employers give analysts read only access.

Many databases, one language

PostgreSQL, MySQL, SQL Server, SQLite and BigQuery differ in small details, but the core SQL in this section works on all of them. If an advert names one you have not used, your skills still transfer.

Practising without installing anything

You do not need a powerful computer. Interactive sites such as SQLBolt let you type queries in the browser and see results instantly, lesson by lesson. Online SQLite playgrounds let you create your own small tables. Both work in any browser, even on a phone, though a laptop with a keyboard is more comfortable for long queries.

Goal: run your first SQL queries in the browser.

1. Open sqlbolt.com on your phone or laptop.
2. Complete lessons 1 to 3, typing each query yourself rather than copying.
3. For each lesson, write in a notebook what the query asked and what came back, in one plain sentence.
4. Then write, in plain English, three questions you would like to ask a database about a business you know, such as a pharmacy or a mobile money agent.

Done looks like: three completed SQLBolt lessons, your notes, and three plain English questions you will turn into SQL in the next lessons.