JPQL - EJB 3 in Action - page 450

Listing 11.5. Retrieving all item names using a query root
The code in this listing will produce the following SQL query:
SELECT ITEM_NAME FROM ITEMS
Now that you know how to create a criteria root, let's look at predicate and join expres-
sions.
Expressions
Expressions are used in the SELECT , WHERE , and HAVING clauses. You use expressions
to specify what you want returned by the query or how you want the query constrained. Ex-
pressions are all rooted by the javax.persistence.criteria.Expression<T>
interface. There are several notable subinterfaces including Predicate , Join , and
Path . To get an instance of an expression, you use utility methods on the Criteri-
aBuilder . You'll notice that in many situations you need an expression to create an
expression. For example, consider the method signature on lessThanOrEqualTo of
CriteriaBuilder :
Predicate lessThanOrEqualTo(Expression<? extends Y> x,
Expression<? extends Y> y)
You can see that this method takes two expressions and returns a predicate. Both expres-
sions make use of generics whose type must match—if you're comparing Double , both
must be Double and so on. To reference a column (property on an entity), you construct
a Path . As mentioned previously, a Path is a type of expression and you use the query
root to construct it by passing in the attribute from the meta-model. Because this can be a
bit confusing, let's look at the findByDate method in the next listing.