Coding Interview Preparation on Telegram: “📊 SQL SATURDAY #2

📊 SQL SATURDAY #2 - JOINs Without the Confusion

Two tables this week:


customers                    orders
+----+---------+            +----+-------------+--------+
| id | name    |            | id | customer_id | amount |
+----+---------+            +----+-------------+--------+
| 1  | Alice   |            | 1  | 1           | 250    |
| 2  | Bob     |            | 2  | 1           | 100    |
| 3  | Charlie |            | 3  | 2           | 75     |
+----+---------+            +----+-------------+--------+

Notice Charlie has no orders.

INNER JOIN - only rows that match in both tables:

sql
SELECT c.name, o.amount
FROM customers c
INNER JOIN orders o ON c.id = o.customer_id;

Charlie won't appear - he has no matching order.

LEFT JOIN - all rows from the left table, matched or not:

sql
SELECT c.name, o.amount
FROM customers c
LEFT JOIN orders o ON c.id = o.customer_id;

Charlie appears with amount = NULL.

⚠️ Interview trap: "Find customers with zero orders."

sql
SELECT c.name
FROM customers c
LEFT JOIN orders o ON c.id = o.customer_id
WHERE o.id IS NULL;

People often try WHERE o.amount = 0 here - wrong, that finds orders worth $0, not customers with no orders at all. Filtering on IS NULL after a LEFT JOIN is the pattern to remember.

Which JOIN type trips you up the most? RIGHT and FULL OUTER are coming in a few weeks 👀