What role does "inventory" play?
select bookid, boooktitle, bookauthor,quantityonhand from inventory.books;
it instructs the query engine to join the books table to the inventory schema
select * from dbo.books here dbo is a schema and the inventory is also schema. If we'd like to specify a database we should use db_name.schema_name.table_name
select * from dbo.books
dbo
db_name.schema_name.table_name
it instructs the query engine to find the books table in the inventory database
it instructs the query engine to find the books table in the inventory schema
you only want to see results from books currently in inventory