Explain EXPLAIN AST (children 1)
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 1)
    ExpressionList (children 2)
     Function isNull (children 1)
      ExpressionList (children 1)
       Function multiply (children 1)
        ExpressionList (children 2)
         Identifier a
         Identifier b
     Function isNotNull (children 1)
      ExpressionList (children 1)
       Function multiply (children 1)
        ExpressionList (children 2)
         Identifier a
         Identifier b
