What role does "inventory" play?
select bookid, boooktitle, bookauthor,quantityonhand from inventory.books;
it instructs the query engine to find the books table in the inventory schema
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
you only want to see results from books currently in inventory