为什么“价值太多”？

Question

master@{1.day.ago}

Answer 1

问题是not in：

select MATNR, HKL
from (select MATNR, HKL
      from BESTAND
      where LOK_NR > 1 and BEST_FREI = 0
      union
      select MATNR, HKL
      from BESTAND
      where LOK_NR = 1 and BEST_FREI > 0
     )
WHERE MATNR NOT IN (select MATNR from SONDER_AUFTRAG where GREIFLOK = 'J' )
GROUP BY MATNR, HKL
ORDER BY MATNR, HKL ;

我认为查询可以简化为：

select distinct MATNR, HKL
from BESTAND b
where ((LOK_NR > 1 and BEST_FREI = 0) or
       (LOK_NR = 1 and BEST_FREI > 0)
      ) and
      not exists (select 1
                  from SONDER_AUFTRAG sa
                  where sa.MATNR = b.MATNR and GREIFLOK = 'J'
                 )
order by MATNR, HKL ;

注意：

• 最好删除union，特别是如果or足够。
• union和group by都会删除查询中的重复项。
• not exists优于带有子查询的not in，因为后者不直观地处理NULL值。