Join 3 table to find total cost - oracle

Question

We need to find out total cost for each project in XYZ company (from the project). Total cost is considered to be sum of (unit price * quantity) from invoice details table.
To find out total cost for each project you need join 3 tables in the query: XYZ_PROJECTS with XYZ_INVOICES (using PROJECT_NUMBER) and XYZ_INVOICES with XYZ_INVOICE_DETAILS (using INVOICE_NUMBER).
Go to SQL WorkshopSQL Commands. Write and execute a “select” query to select project number, project name from XYZ_PROJECTS, sum(c.qty * c.Unit_price) from XYZ_INVOICE_DETAILS. (You need to add “group by project_number, project_name” at the end of the query since SUM aggregate function is used). Run the query. Your screen should like as follows
I used this to join table but I do not know how to join 3 table at the same time.
select *
from XYZ_PROJECTS c
join XYZ_INVOICES do on c.PROJECT_NUMBER = do.PROJECT_NUMBER
select *
from XYZ_INVOICES i
join XYZ_INVOICE_DETAILS do on i.INVOICE_NUMBER = do.INVOICE_NUMBER
Maybe I do it wrong. Please kindly help me.

Answer 1

select c.project_number, c.project_name, sum(do.qty * do.Unit_price)
from XYZ_PROJECTS c
join XYZ_INVOICES i
on c.PROJECT_NUMBER = i.PROJECT_NUMBER
join XYZ_INVOICE_DETAILS do
on i.INVOICE_NUMBER = do.INVOICE_NUMBER
group by c.project_number, c.project_name
If you need to include zero cost projects who do not have any invoices yet, use left outer join to keep all rows from XYZ_PROJECTS:
select c.project_number, c.project_name, sum(do.qty * do.Unit_price)
from XYZ_PROJECTS c
left join XYZ_INVOICES i
on c.PROJECT_NUMBER = i.PROJECT_NUMBER
left join XYZ_INVOICE_DETAILS do
on i.INVOICE_NUMBER = do.INVOICE_NUMBER
group by c.project_number, c.project_name