@Test
public void testUnion() {

  Parser<Relation> parser = RelationParser.union(TABLE);
  TerminalParserTest.assertParser(parser, "a", table("a"));
  TerminalParserTest.assertParser(parser, "a union b", new UnionRelation(table("a"), false, table("b")));
  TerminalParserTest.assertParser(parser, "a union all b union (c)",
      new UnionRelation(
          new UnionRelation(table("a"), true, table("b")),
          false, table("c")
      )
  );
  TerminalParserTest.assertParser(parser, "a union all (b union (c))",
      new UnionRelation(
          table("a"),
          true,
          new UnionRelation(table("b"), false, table("c"))
      )
  );
}