* A *!SingleQuery* expression can be used to search for entities of a _Trait_ or _Class_.
Entities can be filtered based on a 'Where Clause' and Entity Attributes can be retrieved based on a 'Select Clause'.
* An Entity Graph can be traversed/joined by combining one or more !SingleQueries.
* An attempt is made to make the expressions look SQL like by accepting keywords "SELECT",
"FROM", and "WHERE"; but these are optional and users can simply think in terms of Entity Graph Traversals.
* The transitive closure of an Entity relationship can be expressed via the _Loop_ expression. A
_Loop_ expression can be any traversal (recursively a query) that represents a _Path_ that ends in an Entity of the same _Type_ as the starting Entity.
* The _!WithPath_ clause can be used with transitive closure queries to retrieve the Path that
connects the two related Entities. (We also provide a higher level interface for Closure Queries
see scaladoc for 'org.apache.atlas.query.ClosureQuery')
* GROUPBY is optional. Group by can be specified with aggregate methods like max, min, sum and count. When group by is specified aggregated results are returned based on the method specified in select clause. Select expression is mandatory with group by expression.
* ORDERBY is optional. When order by clause is specified, case insensitive sorting is done based on the column specified.
For sorting in descending order specify 'DESC' after order by clause. If no order by is specified, then no default sorting is applied.
* LIMIT is optional. It limits the maximum number of objects to be fetched starting from specified optional offset. If no offset is specified count starts from beginning.
* There are couple of Predicate functions different from SQL:
* _is_ or _isa_can be used to filter Entities that have a particular Trait.
* _has_ can be used to filter Entities that have a value for a particular Attribute.
* Any identifiers or constants with special characters(space,$,",{,}) should be enclosed within backquote (`)
---++++ DSL Examples
For the model,
Asset - attributes name, owner, description
DB - supertype Asset - attributes clusterName, parameters, comment
