All language subtitles for 003 Understand JPA Query and Named Query

Before implementing the listAll() and count() methods of the UserDAO class

Let's understand how to execute a query in JPA

because we need to use query in these methods

This is a code examples that shows you how to execute a query that returns a single value

a single result

you can see the query here

SELECT u from User u where u.email = 'billjoy@gmail.com'

this is not standard SQL

it's the JPQL or Java Persistence Query Language

which is similar to a standard SQL

this JPQL uses object-oriented syntax which is more convenient for programmers

you can see

you can see

u.email here is object-oriented syntax

it accesses the email field of the User object named u

and the next statement creates a Query object from the query string

and the getSingleResult() method executes the query and returns a single result

such as an integer value, a String or an object

and here's the second example that executes a query that returns a collection of values

such as a list of mapped object from the database

you can see

we use the getResultList() method of the Query object

this code executes the query that retrieves all rows from the table user

and returns the result as a collection - a list of mapped User objects

is very cool, isn't it?

A named query in JPA is a query string which can be embedded with...

in a model class using the annotation...

you see

the annotation @NamedQueries and @NamedQuery here

you can see

this annotation here

this is the content of the query

you see

JPQL

and...

this is the name of the query: Users.findAll

you can give it any name

so the query can be referred by its name as shown in the code below

you see

From the entity manager we create a named query

and refer to a named query by its name

this line creates a Query object that refers to a query by its name

JPA automatically loads named queries that are embedded in model classes

so you can call them in your code

the benefit of using named queries is that you can centralize related queries in one place

typically in the model classes you can manage the queries easily when your code grows and become more complex

as you can see

The @NamedQueries annotation allows multiple @NamedQuery to be listed here

so you can specify multiple named queries in a model class