public Set<Analysis.AliasedDataSource> extractDataSources(final AstNode node) {
    new Visitor().process(node, null);
    return getAllSources();
  }
@Test
  public void shouldThrowIfInnerJoinSourceDoesNotExist() {
    // Given:
    final AstNode stmt = givenQuery("SELECT * FROM TEST1 JOIN UNKNOWN"
        + " ON test1.col1 = UNKNOWN.col1;");

    // When:
    final Exception e = assertThrows(
        KsqlException.class,
        () -> extractor.extractDataSources(stmt)
    );

    // Then:
    assertThat(e.getMessage(), containsString(
        "UNKNOWN does not exist."));
  }