SelectWithUnionQuery (children 1)
 ExpressionList (children 1)
  SelectQuery (children 2)
   ExpressionList (children 4)
    Identifier a1
    Identifier a2
    Identifier b1
    Identifier b2
   TablesInSelectQuery (children 2)
    TablesInSelectQueryElement (children 1)
     TableExpression (children 1)
      TableIdentifier tab1 (alias first)
    TablesInSelectQueryElement (children 2)
     TableExpression (children 1)
      TableIdentifier tab2 (alias second_)
     TableJoin (children 1)
      Function equals (children 1)
       ExpressionList (children 2)
        Identifier first.b1
        Identifier second_.a2