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To help with this DomUI defines a generic query layer called the QCriteria layer. For people that know Hibernate or JPA: this is the same idea as its "Criteria API", but with a cleaner implementation and some important differences. Lets see how we do queries using this layer.

Where is the code

The examples in here can be found in the "tutorial" module. Get the domui code from github, then find the module at examples/tutorial. The tutorial project is a standalone project configured with Maven, and you can open it in IntelliJ by just opening the directory and importing the POM file.

My first query

To do a QCriteria query you need two things:

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This is the equivalent of "select * from ALBUM", i.e. a select for all records in the database. To actually add a restriction, so called because it restricts the number of results, we add one of the comparator methods on it, like this:

public class CritTest {
 public static void main(String[] args) throws Exception {
   QCriteria<Album> q = QCriteria.create(Album.class);
   q.eq("title", "Nevermind");

   System.out.println("Q= " + q);
 }
}

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So far we only printed output. Lets actually do a query. Change the code to look likeLet's make a new class "Criteria1":

package to.etc.domui.tutorial.criteria;

import to.etc.domui.derbydata.db.Album;
import to.etc.webapp.query.QCriteria;

/**
 * @author <a href="mailto:jal@etc.to">Frits Jalvingh</a>
 * Created on 2-4-18.
 */
public class CritTest {
    public static void main(String[] args) throws Exception {
        QCriteria<Album> q = QCriteria.create(Album.class);
        q.eq("title", "Nevermind");

        System.out.println("GotQ= " + a.getTitle()q);
    }
}

This code does the following:

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The parameter is actually a JDBC parameter (the ?) so SQL injection is not possible.

The result of the class is as we would expect:

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Combinators: and and or in QCriteria queries

We have seen that when we use multiple comparator methods on a QCriteria it will combine them with the "and" operator. So using "and" is easy ;-) But how do we make an "or"? Before we can answer that we need to explain something.

The QCriteria instance you get from the create operation is an instance of a QRestrictor<T>. The QRestrictor "restricts" the result set by adding stuff to the where clause. This QRestrictor has a "combination" mode which is "AND" - so all comparator methods added to it are combined with AND's. To make an OR you must create a QRestrictor that uses a combination mode of OR. You get such a thing as the result of the or() call: a QRestrictor<T> where every comparator method called is combined with or. For example: the query

where year=2010 or year=2011

is coded as follows:

QCriteria<Album> q = QCriteria.create(Album.class);
QRestrictor<Album> or = q.or();
or.eq("year", 2010);
or.eq("year", 2011);

When you add new conditions to the "parent" QRestrictor (q) they will be combined with and:

QCriteria<Album> q = QCriteria.create(Album.class);
QRestrictor<Album> or = q.or();
or.eq("year", 2010);
or.eq("year", 2011);
q.ilike("name", "A%");

will result in:

where (year=2010 or year=2011) and name ilike 'A%'

To create something like

where (year=2010 and name ilike 'A%') or (year=2011 and name ilike 'Z%')

you would code:

QCriteria<Album> q = QCriteria.create(Album.class);
QRestrictor<Album> or = q.or();

or.and().eq("year", 2010).ilike("name", "A%");
or.and().eq("year", 2011).ilike("name", "Z%");

The "or.and()" method call returns another restrictor that uses and again.

Building trees

When you use QCriteria you are building an expression tree. The tree has nodes that represent operations and leaves that represent subexpressions or terms in the expression. For instance, the expression tree for:

(2 + 2) + (2 + 2) + (3 + 3)

is:

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QCriteria creates such a tree for boolean expressions, and important parts in it are the and and or operations:

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