# QueryEngine

The `QueryEngine` handles the transformation of the `ask` query language into a `SQL` construct and is also
responsible to return query results from the `SQL` back-end with the help of the following components:

- The `QuerySegmentListBuilder` transforms `ask` descriptions into individual `QuerySegment`'s (aka `QuerySegmentList`)
- The `DescriptionInterpreter` interface describes classes that are responsible to interpret a specific
  `Description` object and turn it into an abstract `SQL` construct (a `QuerySegment`)
- The `QuerySegmentListProcessor` flattens and transforms a list of `QuerySegment`'s into a non-recursive
  tree of `SQL` statements (including resolving of property/category hierarchies)
- The `ConceptQueryResolver` encapsulates query processing of a concept description in connection
  with the `ConceptCache` class

## Overview

![image](https://cloud.githubusercontent.com/assets/1245473/10050078/ca42ff12-621a-11e5-84c3-5fd04d945c6c.png)

### Examples
```php
/**
 * Equivalent to [[Category:Foo]]
 */
$classDescription = new ClassDescription(
	new DIWikiPage( 'Foo', NS_CATEGORY )
);

/**
 * Equivalent to [[:+]]
 */
$namespaceDescription = new NamespaceDescription(
	NS_MAIN
);

/**
 * Equivalent to [[Foo::+]]
 */
$someProperty = new SomeProperty(
	new DIProperty( 'Foo' ),
	new ThingDescription()
);

/**
 * Equivalent to [[:+]][[Category:Foo]][[Foo::+]]
 */
$description = new Conjunction( [
	$namespaceDescription,
	$classDescription,
	$someProperty
] );
```
```php
$query = new Query( $description );
$query->setLimit( 10 );

$sqlStorefactory = new SQLStoreFactory(
  new SQLStore()
);

$queryEngine = $sqlStorefactory->newMasterQueryEngine();
$queryResult = $queryEngine->getQueryResult( $query );
```