User's query
* Single table query
* There are too few fields to display on the page compared to the database
select new Person(pid,pname) from Person;
* The fields to be displayed on the page are similar to the fields of the database
from Person
* Multi-table query
Customer and Order Form
* Display customer information in a page, there is a hyperlink in the customer information, click on the hyperlink
Display information about the customer's order in another page
* 后台:"from Customer"
Because the default loading mode for one-to-many collections is lazy loading, when the above hql statement is executed
The customer's order is not loaded, so only customer information can be loaded on the first page
* When the hyperlink is clicked, the background "from Order o where o.customer.cid=?"
Using such an hql statement, you can load order information
* Display customer information and customer order information in one page
Using an eager left outer join, only one SQL statement is issued
Three tables (one-to-many, many-to-many) can be done by pressing the left outer join of the three tables
If there are multiple tables, the fields on the page are too different from the fields in the database
"from User";
user.getDepartment().getDname()
user.getPosts()
pname