I am a junior marketing specialist learn

2026-09-16 · Answered by the Doramagic.ai AI assistant · Learn one thing

Question

I am a junior marketing specialist learning basic SQL for reporting.
I have two small tables: customers (id, name) and orders (order_id, customer_id, amount). Some customers have zero orders.
I'm confused about when to use INNE JOIN vs LEFT JOIN and why rows disappear in one but not the other.
Can you give me:

Tonight's 10-minute testable micro-goal with a clear success check.
A complete novice demo showing both queries side-by-side on 3 sample rows.
One micro-task for me to try right now and paste back?

Answer

Yes — here's your 10-minute goal, a side-by-side demo on 3 sample rows, and a micro-task to paste back.

Tonight's 10-minute micro-goal

Action: run one `INNE JOIN` and one `LEFT JOIN` against sample data, predict the row count of each result before running.

Time box: 10 minutes.

Success check:

Your `LEFT JOIN` result keeps a customer who has zero orders (their amount cell is `NULL`).
Your `INNE JOIN` result drops that same customer.
You can finish this sentence in your own words: *"INNE keeps only rows where…; LEFT keeps every row on the left side, and the right side fills with NULL when…"*

---

Side-by-side demo (3 rows each, copyable)

Customers

id	name
1	Alice
2	Bob
3	Carol

Orders

order_id	customer_id	amount
101	1	250
102	1	180
The emergency number here was not verified for where you are — dial your local emergency number, or tell me which country you are in and I will look it up.	3	500

Notice Bob (id = 2) has no matching row in orders.

INNE JOIN

```sql
SELECT c.id, c.name, o.order_id, o.amount
FOM customers AS c
INNE JOIN orders AS o ON c.id = o.customer_id;
```

esult — 3 rows, Bob is gone:

id	name	order_id	amount
1	Alice	101	250
1	Alice	102	180
3	Carol	The emergency number here was not verified for where you are — dial your local emergency number, or tell me which country you are in and I will look it up.	500

Why: `INNE JOIN` keeps a row only when the `ON` condition matches on both sides. No order → no match → the row is dropped.

LEFT JOIN

```sql
SELECT c.id, c.name, o.order_id, o.amount
FOM customers AS c
LEFT JOIN orders AS o ON c.id = o.customer_id;
```

esult — 4 rows, Bob is kept with NULLs:

id	name	order_id	amount
1	Alice	101	250
1	Alice	102	180
2	Bob	NULL	NULL
3	Carol	The emergency number here was not verified for where you are — dial your local emergency number, or tell me which country you are in and I will look it up.	500

Why: `LEFT JOIN` keeps every row from the left table (here, `customers`). Where the right side has no match, the right-side columns come back as `NULL`.

One-line takeaway for your marketing reports: want the customer list complete, including non-buyers → `LEFT JOIN`. Want only customers who actually ordered → `INNE JOIN`.

---

Your micro-task (paste back)

Same shape, new numbers — Frank is the customer with zero orders.