Cypher support in TQL

xGT TQL Queries (MATCH)

xGT's TQL language includes a subset of the Cypher language. This document is focused on describing the particular details of TQL's supported Cypher subset for users familiar with neo4j's Cypher language.

xGT supports a restricted form of the Cypher query language to enable the exploration of property graph datasets. The principal Cypher command supported by xGT is the read-only MATCH command with constraints. We recommend that you familiarize yourself with the Cypher language and in particular the syntactic components of the MATCH command.

A MATCH query in TQL consists typically of three parts:

  1. The graph pattern description.
  2. Constraints on properties & object identities.
  3. Specification on how the answer set should be produced.

The graph pattern description consists of a list of steps over graph objects in xGT. Each step in the graph pattern corresponds to either a vertex or edge match. A pattern can consist of a sequence of steps which are fully connected in the graph or it can consist of multiple (comma separated) of those sequences which are connected as a graph pattern by intermediate vertices.

Constraints on properties and object identities are expressed as part of a MATCH query using a WHERE clause. The WHERE clause can contain conditions based on property values of vertices and/or edges (a.year > 1980), as well as identity comparison between vertices or edges (a <> b).

The answer set specification consists of what results should be produced from the MATCH query. Including which properties of which entities should be reported back to the user, as well as solution modifiers, such as sorting the results based on some of the columns, reporting only unique results or aggregating information over columns.

Details on supported Cypher constructs in TQL

The fundamental concepts used in the Cypher subset supported by TQL are the following:

Examples of TQL queries

We illustrate TQL subset of Cypher with two examples:

Example 1

MATCH (a:Person)-[b:WorksFor]->[c:Company]
WHERE 100 <= a.PersonID AND a.PersonID <= 150
RETURN a.Name, c.Name

In this simple example, we want to match people with an id between 100 and 150 for any company in our graph. We use labels (textual identifier after the ":" on each step) to identify which xGT object to use for that particular graph step. We use variables to capture matched information in order to constrain the results (a.PersonID <= 150), as well as to indicate what values should be inserted into the results table. Notice that the Cypher graph steps use different syntax ("( )") and ("[ ]") for vertices and edges, respectively. Before execution, the query is type checked against the declared xGT objects present in the database. In this case, the xGT database must contain a WorksFor edge frame. This edge frame must connect vertices belonging to the Person frame to vertices belonging to the Company frame.

This particular query example would also be valid in neo4j's Cypher, given the existence of appropriate data with the indicated Cypher labels.

Example 2

MATCH (a)-[:TrianglesEdge]->(b:)-[]->(c:)-[:TrianglesEdge]->(a)
WHERE a <> b AND b <> c
AND c <> a
RETURN a.UniqueID, b.UniqueID, c.UniqueID

This more complex example finds triangles in the graph. In this case, the frame label for the middle edge step is not given and must be inferred. The return clause indicates that we want the UniqueID values (endpoints) of all triangles found in the graph.

Type inference in TQL queries

TQL interprets Cypher labels as a type annotation corresponding to the unique name of a vertex or edge frame. The principal difference between Cypher and TQL is that TQL supports only a single label per graph step and it must correspond to a known xGT graph object.

We can say that a vertex belonging to the Person vertex frame is of type Person and an edge belonging to the WorksFor edge frame is of type WorksFor.

Our TQL compiler uses type inference to enable the elision of those annotations in many cases so that TQL queries can be written in a manner similar to their Cypher analogues. The TQL compiler tries to automatically deduce what the type annotation should be for different graph objects. In particular, given a type annotation for an edge step "[ :edge ]", the compiler can deduce the types of the two vertex steps on either side of the edge step: (a)-[:edge]->(b). In this case, the type annotation for the "a" and "b" vertex steps is not needed.

Edge type annotations can be inferred for edge steps when the preceding and succeeding vertex steps have type annotations and those vertex frames are only used in the declaration of a single edge frame. If vertices of a frame are connected by edges belonging to multiple edge frames, then an unannotated edge step could be ambiguous and must have its own type annotation: for example, if we have edge types worksFor and consumerOf with vertex types Person and Company for both, then the following unannotated query is ambiguous for the edge step "b": (a:Person)-[b]->(c:Company).

Multiple unannotated vertex and edge steps are supported in a query because types are inferred and propagated from one step to another by the compiler, e.g. (a)-[:edge]->(b)-[]->(c). Type inference continues iteratively until all graph pattern steps in the query have been "typed" correctly or the compiler has detected an error.

Limitations on TQL's Cypher support