What are PetroChina's assets?
select assets_in_billion from company where name = "PetroChina"
How many employees are there?
select count(*) from employment as t1 join people as t2 on t1.people_id = t2.people_id join company as t3 on t1.company_id = t3.company_id where t3.name = "PetroChina"
Tell me the names of the companies and the number of employees they have.
select t3.name, count(*) from employment as t1 join people as t2 on t1.people_id = t2.people_id join company as t3 on t1.company_id = t3.company_id group by t3.name