public Set<Analysis.AliasedDataSource> extractDataSources(final AstNode node) {
    new Visitor().process(node, null);
    return getAllSources();
  }
@Test
  public void shouldThrowIfLeftJoinSourceDoesNotExist() {
    // Given:
    final AstNode stmt = givenQuery("SELECT * FROM UNKNOWN JOIN TEST2"
        + " ON UNKNOWN.col1 = test2.col1;");

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

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