Oracle Tax Rate Code Query

   SELECT DISTINCT tax_rate_code
     FROM ZX_RATES_B a, ZX_REGIMES_B b
    WHERE     a.tax_regime_code = b.tax_regime_code
          AND (   a.effective_to IS NULL
               OR TRUNC (a.effective_to) >= TRUNC (SYSDATE))
          AND (   b.effective_to IS NULL
               OR TRUNC (b.effective_to) >= TRUNC (SYSDATE));