Questions about correlated subqueries


i am learning SQL with Mosh now.

for the course : Correlated Subqueries, there is one making me confused.

–Get invoices that are larger than the
– Client’s average invoice amount.

the coding:
USE sql_invoicing;
select *
from invoices i
where invoice_total > (
select AVG(invoice_total)
from invoices
where client_id = i.client_id

My question:
How to understand the last line? why shall give alliance without connecting two tables?

By the way
is there any living exchange chatting group recommend?

thank you very much.


I don’t really understand the second part of your question but client_id = i.client_id says that we only want to include rows in the inner query that have the same client_id as the row in the outer query.

thank you SAM.

the second question is: is there any group chat, like whatsapp to discuss questions about the course?