· beginner
Joining Data in SQL
Real data lives in several tables at once. Learn to combine them deliberately, keep the rows you meant to keep, and spot the joins that quietly inflate your numbers.
Written in house, from the work rather than from a syllabus. Every example is one we would use ourselves, and every exercise is checked against the same engine that grades your answer.
What you’ll learn
- Explain why data is split across tables and how keys reconnect it
- Choose between INNER, LEFT, RIGHT and FULL joins on purpose
- Join three or more tables without losing rows by accident
- Use a self-join to compare rows within one table
- Recognise fan-out and stop a join from double counting
Before you start
- · SQL Foundations, or comfort with SELECT, WHERE and GROUP BY
- · No knowledge of joins assumed