BitWithBite
SQL Worksheet
🗃 JOINs: Combining Tables
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After this worksheet you can
INNER JOIN two tablesLEFT JOIN and NULLsJoin on foreign keysAlias tables📚 Quick Recap
Relational databases split data into multiple tables to avoid redundancy — students in one table, courses in another, enrollments in a third. JOINs bring that data back together at query time. Mastering INNER JOIN, LEFT JOIN, and when to use each is the single biggest skill jump in your SQL journey.
🧠 Section A · Concept Check ● BEGINNER 4 × 1 = 4
1Rows matching in BOTH tables:
2Keep all left rows even without match:
3Join condition uses:
4Unmatched LEFT JOIN columns show:
🧮 Section B · Problem Solving ● INTERMEDIATE 2 + 3×3 = 11
5JOIN orders users.id = orders.user_id;
6A key points to another table's primary key.
7List each order with its customer name.
8All users and their orders, including users with none.
9Find users with NO orders.
🚀 Section C · Challenge ● CHALLENGE 5
10INNER vs LEFT join in one line?
💭 Reflection — the most useful thing I learned:
A ___/4 B ___/11 C ___/5 Total ___/20
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✂ answer key — fold or cut before handing out
1-A 2-B 3-A 4-C | 5 ON 6 foreign 7 = SELECT o.id, u.name FROM orders o INNER JOIN users u ON u.id = o.user_id; 8 = LEFT JOIN orders o ON o.user_id = u.id 9 = LEFT JOIN ... WHERE o.id IS NULL | 10 = INNER = matches only; LEFT = all left rows + NULLs for no match