Adding an unused table after FROM changes retrieved data - sql

Question

I am referring to the Chinook database, which i am using to learn SQLite.
This query retrieves the number of invoices for each CustomerId, as I wanted:
select i.customerid, count(i.invoiceid)
from invoices as i
group by i.customerid
returns:
+------------+--------------------+
| CustomerId | count(i.invoiceid) |
+------------+--------------------+
| 1 | 7 |
| 2 | 7 |
| 3 | 7 |
...
But as i was building a more complex query i observed something that i cannot explain:
select i.customerid, count(i.invoiceid)
from invoices as i, customers as c
group by i.customerid
returns:
+------------+--------------------+
| CustomerId | count(i.invoiceid) |
+------------+--------------------+
| 1 | 413 |
| 2 | 413 |
| 3 | 413 |
...
Turns out 413 = 7 * 59, and 59 is the number of distinct CustomerID's.
There must be some fundamental SQL behavior that i am misunderstanding here, because I would expect no difference by adding "customers as c" in the "from" clause, since I am not using it yet. Can anyone enlighten me here on what is happening?

Answer 1

Never use commas in the FROM clause. Only use proper, explicit, standard, readable JOIN syntax.
Your query is producing a Cartesian product of the rows in the two tables. Then your aggregation counts the number of rows, for each customer, in the Cartesian product.
You need something like this:
select i.customerid, count(i.invoiceid)
from invoices i join
customers c
on i.customerid = c.customerid
group by i.customerid

Answer 2

You are performing a cross join which is the cartesian product of the rows of your 2 tables. You were right about the origin of the 413 value.
With a cross join, if table A has 5 rows and table B has 7 rows, it will produce a result of 5 * 7 = 35 rows.
When joining tables, you then need to add a join condition which will filter unrelated rows (cross joins are rarely what you want):
SELECT i.customerid, count(i.invoiceid)
FROM invoices as i, customers as c
WHERE i.customerid = c.id -- join condition
GROUP BY i.customerid
But the recommended syntax for join is explicit (no comma):
SELECT i.customerid, count(i.invoiceid)
FROM invoices as i
JOIN customers as c -- explicit join
ON i.customerid = c.id -- join condition
GROUP BY i.customerid
But this will perform an INNER JOIN by default which requires that rows from invoices table matches at least 1 row from customers, and vice-versa.
If you still want to display customers with 0 invoices, you need to use LEFT JOIN to keep rows from the left table (first one of the FROM clause) even if they have no match with the right table:
SELECT i.customerid, count(i.invoiceid)
FROM invoices as i
LEFT JOIN customers as c -- keep customers without invoices
ON i.customerid = c.id -- join condition, unchanged
GROUP BY i.customerid