src/main/asciidoc/jdoql.adoc

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== JDOQLanchor:JDOQL[]

JDO defines ways of querying objects persisted into the datastore. It
provides its own object-based query language (JDOQL). JDOQL is designed
as the Java developers way of having the power of SQL queries, yet
retaining the Java object relationship that exist in their application
model. A typical JDOQL query may be set up in one of 2 ways. Here's an
example

....
Declarative JDOQL :
Query q = pm.newQuery(mydomain.Person.class, "lastName == \"Jones\" && age < age_limit");
q.declareParameters("double age_limit");
List results = (List)q.execute(20.0);

Single-String JDOQL :
Query q = pm.newQuery("SELECT FROM mydomain.Person WHERE lastName == \"Jones\"" +
                      " && age < :age_limit PARAMETERS double age_limit");
List results = (List)q.execute(20.0);
....

So here in our example we select all "Person" objects with surname of
"Jones" and where the persons age is below 20. The language is intuitive
for Java developers, and is intended as their interface to accessing the
persisted data model. As can be seen above, the query is made up of
distinct parts. The class being selected (the SELECT clause in SQL), the
filter (which equates to the WHERE clause in SQL), together with any
sorting (the ORDER BY clause in SQL), etc.

Before giving details on JDOQL, you can download a quick reference guide
link:jdoql_quickref.pdf[here]

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anchor:singlestring[]

=== Single-String JDOQLanchor:Single-String_JDOQL[]

In traditional (declarative) JDOQL (JDO 1.0) it was necessary to specify
the component parts (filter, candidate class, ordering, etc) of the
query using the mutator methods on the Query. In JDO 2 you can now
specify it all in a single string. This string has to follow a
particular pattern, but provides the convenience that many people have
been asking for. The pattern to use is as follows

....
SELECT [UNIQUE] [<result>] [INTO <result-class>]
        [FROM <candidate-class> [EXCLUDE SUBCLASSES]]
        [WHERE <filter>]
        [VARIABLES <variable declarations>]
        [PARAMETERS <parameter declarations>]
        [<import declarations>]
        [GROUP BY <grouping>]
        [ORDER BY <ordering>]
        [RANGE <start>, <end>]
....

The "keywords" in the query are shown in UPPER CASE but can be in
_UPPER_ or _lower_ case.

Lets give an example of a query using this syntax

....
SELECT UNIQUE FROM org.datanucleus.samples.Employee ORDER BY departmentNumber
....

so we form the parts of the query as before, yet here we just specify it
all in a single call.

=== Accessing Fieldsanchor:Accessing_Fields[]

In JDOQL you access fields in the query by referring to the field name.
For example, if you are querying a class called _Product_ and it has a
field "price", then you access it like this

....
Query query = pm.newQuery(mydomain.Product.class, "price < 150.0");
....

In addition to the persistent fields, you can also access "public static
final" fields of any class. You can do this as follows

....
Query query = pm.newQuery(mydomain.Product.class, 
       "taxPercent < mydomain.Product.TAX_BAND_A");
....

So this will find all products that include a tax percentage less than
some "BAND A" level. Where you are using "public static final" fields
you can either fully-qualify the class name or you can include it in the
"imports" section of the query (see later).

=== Data types : literalsanchor:Data_types_:_literals[]

JDOQL supports the following literals: IntegerLiteral,
FloatingPointLiteral, BooleanLiteral, CharacterLiteral, StringLiteral,
and NullLiteral.

=== Operators precedenceanchor:Operators_precedence[]

The following list describes the operator precedence in JDOQL.

[arabic]
. Cast
. Unary ("~") ("!")
. Unary ("+") ("-")
. Multiplicative ("*") ("/") ("%")
. Additive ("+") ("-")
. Relational (">=") (">") ("<=") ("<") ("instanceof")
. Equality ("==") ("!=")
. Boolean logical AND ("&")
. Boolean logical OR ("|")
. Conditional AND ("&&")
. Conditional OR ("||")

=== Concatenation Expressionsanchor:Concatenation_Expressions[]

The concatenation operator(+) concatenates a String to either another
String or Number. Concatenations of String or Numbers to null results in
null.

=== Example 1 - Use of Explicit Parametersanchor:Example_1_-_Use_of_Explicit_Parameters[]

Here's a simple example for finding the elements of a class with a field
below a particular threshold level. Here we pass in the threshold value
(_limit_), and sort the output in order of ascending price.

....
Declarative JDOQL :
Query query = pm.newQuery(mydomain.Product.class,"price < limit");
query.declareParameters("double limit");
query.setOrdering("price ascending");
List results = (List)query.execute(150.00);

Single-String JDOQL :
Query query = pm.newQuery("SELECT FROM mydomain.Product WHERE " +
                "price < limit PARAMETERS double limit ORDER BY price ASCENDING");
List results = (List)query.execute(150.00);
....

For completeness, the class is shown here

....
class Product
{
    String name;
    double price;
    java.util.Date endDate;
    ...
}

<jdo>
    <package name="mydomain">
        <class name="Product">
            <field name="name">
                <column length="100" jdbc-type="VARCHAR"/>
            </field>
            <field name="abreviation">
                <column length="20" jdbc-type="VARCHAR"/>
            </field>
            <field name="price"/>
            <field name="endDate"/>
        </class>
    </package>
</jdo>
....

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=== Example 2 - Use of Implicit Parametersanchor:Example_2_-_Use_of_Implicit_Parameters[]

Let's repeat the previous query but this time using _implicit_
parameters.

....
Declarative JDOQL :
Query query = pm.newQuery(mydomain.Product.class,"price < :limit");
query.setOrdering("price ascending");
List results = (List)query.execute(150.00);

Single-String JDOQL :
Query query = pm.newQuery("SELECT FROM mydomain.Product WHERE " +
                "price < :limit ORDER BY price ASCENDING");
List results = (List)query.execute(150.00);
....

So we omitted the declaration of the parameter and just prefixed it with
a colon (:)

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=== Example 3 - Comparison against Datesanchor:Example_3_-_Comparison_against_Dates[]

Here's another example using the same Product class as above, but this
time comparing to a Date field. Because we are using a type in our
query, we need to _import_ it ... just like you would in a Java class if
you were using it there.

....
Declarative JDOQL :
Query query = pm.newQuery(domain.Product.class,
                          "endDate > best_before_limit");
query.declareImports("import java.util.Date");
query.declareParameters("Date best_before_limit");
query.setOrdering("endDate descending");
Collection results = (Collection)query.execute(my_date_limit);

Single-String JDOQL :
Query query = pm.newQuery("SELECT FROM mydomain.Product " +
                "WHERE endDate > best_before_limit " +
                "PARAMETERS Date best_before_limit " +
                "import java.util.Date ORDER BY endDate DESC");
List results = (List)query.execute(my_date_limit);
....

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=== Example 4 - Instanceofanchor:Example_4_-_Instanceof[]

This example demonstrates use of the "instanceof" operator. We have a
class A that has a field "b" of type B and B has subclasses B1, B2, B3.
Clearly the field "b" of A can be of type B, B1, B2, B3 etc, and we want
to find all objects of type A that have the field "b" that is of type
B2. We do it like this

....
Declarative JDOQL :
Query query = pm.newQuery(mydomain.A.class);
query.setFilter("b instanceof mydomain.B2");
List results = (List)query.execute();

Single-String JDOQL :
Query query = pm.newQuery("SELECT FROM mydomain.A WHERE b instanceof mydomain.B2");
List results = (List)query.execute();
....