Ok, I have three tables: *theater*, *spectacles* and *tickets*. A *theater* has one or more *spectacles* and for a *spectacle* were sold one or more *tickets** So, **id_theater** is foreign key in **TABLE SPECTACLES** and **id_spectacle** is foreign key in **TABLE TICKETS**. In table TICKETS, I have **ticket_price** and **id_ticket** and with **count(id_ticket)** I calculate the number of tickets sold for spectacles. And each theater has one or more spectacles. The value for spectacles is calculated as: **count(tickets.id_ticket) * tickets.ticket_price**. After I calculate the value for spectacles, I need to gather the value of spectacles to find the each value of theater and for this I use **SUM()**.

**I need to write a query where I need to calculate the total value obtained by each theater and to be sort by value. For this I need to use the SUM() function.** I tried so many different options, but I have different errors. When I tried to run this, this error appeared :

`SQL Error: ORA-00937: not a single-group group function `

`SELECT theater.id_theater, SUM(count(tickets.id_ticket) * tickets.id_ticket) total_theater FROM spectacles, theater GROUP BY theater.id_theater ORDER BY total_theater DESC; `

For this problem I must use **SUM**, but I really don’t know what do to at this point and would appreciate any help.