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# 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
### 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 );
```
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