Handle Union in opencypher2relalg compilation

[jmarton]

• add Union as BetaOperator to EMF model

• handle Union in cyp AST -> relalg compilation

• use different VariableFactories fo each singleQuery (i.e. those that are unioned together)

• make distinction between UNION ALL/UNION

  match (n:A)
  return n
  union
  match (n:D)
  return n
  

• add requirement: variable aliases need to be matched (see also: Handle expression aliases for RETURN and WITH clauses #16)
  otherwise exception is thrown in Neo4j, see:

  CREATE (n1:A{name:"World"}) create (n2:B{whatever:"Universe"});
  MATCH (a:A) RETURN a AS a
  UNION
  MATCH (b:B) RETURN b AS a;
  

  Neo4j: All sub queries in an UNION must have the same column names

[jmarton]

Using different variablefactories for each singleQuery is done in commit 458f942

[szarnyasg]

This does not work for some reason.

[jmarton]

@szarnyasg please give me a query leading to failing/erroneous compilation.

[szarnyasg]

Sorry, my mistake - all failing queries only had RETURN clauses (i.e. RETURN ... UNION RETURN ...), which is currently not supported.