Your First T-SQL Query

What is T-SQL, and why does it matter in Africa?

T-SQL (Transact-SQL) is the language that runs inside Microsoft SQL Server — the database engine used by thousands of enterprises across Nigeria, Kenya, South Africa, Ghana, and beyond. Banks, telecoms, fintech platforms, healthcare systems, logistics companies: they all store their data in SQL Server, and they all need people who can query it.

When you know T-SQL, you can answer business questions directly from the data. No waiting for an engineer. No exporting to Excel. You open a query window, write a few lines, and the answer is right there.

This lesson covers the two most fundamental tools in that process: SELECT and WHERE.

Our dataset: PalmPay-style mobile money transactions

Imagine you're a junior data analyst at a Nigerian mobile money company — similar to PalmPay or OPay. The transactions team hands you access to a table called mobile_transactions. It looks like this:

Each row is one transaction. Your job: write queries to extract exactly the data the business needs.

Part 1 — SELECT: Choose your columns

Every T-SQL query starts with SELECT. It tells the database which columns you want back. The simplest possible query retrieves everything:

SELECT *
FROM   mobile_transactions;

The * is a wildcard — it means "every column." This returns all 7 columns for every row in the table. In a real fintech database that might be millions of rows. You'd never run SELECT * in production. But it's fine to learn with.

More often, you'll name the columns you actually need:

SELECT transaction_id,
       customer_id,
       amount_ngn,
       status
FROM   mobile_transactions;

Part 2 — WHERE: Filter your rows

SELECT controls which columns you see. WHERE controls which rows you see. This is where T-SQL starts to feel powerful — instead of pulling the whole table, you can ask very specific business questions.

Example 1: Only completed transactions

The operations team wants a list of every transaction that went through successfully. You need rows where status = 'completed':

SELECT transaction_id,
       customer_id,
       amount_ngn,
       status
FROM   mobile_transactions
WHERE  status = 'completed';

From our sample data, this returns 4 rows: TXN-10001, TXN-10002, TXN-10004, and TXN-10005. The failed and pending transactions are excluded.

Example 2: High-value transfers

The compliance team flags large transfers for review. Show them every completed transfer above ₦20,000:

SELECT transaction_id,
       customer_id,
       amount_ngn,
       state
FROM   mobile_transactions
WHERE  transaction_type = 'transfer'
  AND  status = 'completed'
  AND  amount_ngn > 20000;

Three conditions, all connected with AND. A row only appears if it satisfies all three. From our sample: only TXN-10005 (₦50,000 transfer from Kano, completed) passes all three filters.

Example 3: All transactions from Lagos or Abuja

The regional team wants data from two states. Use IN — it's cleaner than stacking OR conditions:

SELECT transaction_id,
       customer_id,
       amount_ngn,
       state,
       status
FROM   mobile_transactions
WHERE  state IN ('Lagos', 'Abuja');

This returns 4 rows: TXN-10001, TXN-10003 (Lagos), TXN-10002, TXN-10006 (Abuja). IN is the shorthand for state = 'Lagos' OR state = 'Abuja' — but much easier to read when you have 5+ values.

What you just learned

In one lesson, you can now:

• Use SELECT to pick which columns to return
• Use FROM to specify the table
• Use WHERE to filter rows by value, range, or list
• Chain conditions with AND and IN
• Apply all of this to a real-world business scenario

This is Week 1 of 8 in the T-SQL Mastery program. By Week 4 you'll be writing joins, aggregations, and subqueries against real enterprise datasets. By Week 8, you'll have a portfolio project you can show in interviews.