您可以尝试用JOIN子句替换IN子句

（而且我在带有独特子句的子查询中删除了不带聚集功能的分组）

    SELECT
    SIZE,COUNT(SIZE)
FROM
  ( SELECT ITEM,MIN(SZCUBC) AS CUBE
        FROM XXX.ITEM 
        JOIN XXX.ITEMA ON IAITEM = ITEM 
        JOIN  (
          SELECT DISTINCT SIZEB 
          FROM DB.LOC 
          WHERE BQTY>0
        ) t2 ON t2.SIZEB = DB.SIZECD.SIZE
        LEFT JOIN DB.SIZECD ON ITDHGT*ITDWTH*ITDDTH*QTY <= SZCUBC 
        WHERE IASOHQ >0 
        GROUP BY ITEM
        ) t
LEFT JOIN DB.SIZECD T1 ON SZCUBC = CUBE 

GROUP BY  SIZE

针对每个DB2 docs可能存在相关错误：

  

ON子句不能包含子查询。

因此，请考虑使用WHERE替代品：

...
  (SELECT ITEM,MIN(SZCUBC) AS CUBE
   FROM XXX.ITEM 
   JOIN XXX.ITEMA ON IAITEM = ITEM 
   LEFT JOIN DB.SIZECD ON ITDHGT*ITDWTH*ITDDTH*QTY <= SZCUBC
   WHERE IASOHQ > 0 
     AND SIZE IN (SELECT SIZEB 
                  FROM DB.LOC 
                  WHERE BQTY > 0) 
   GROUP BY ITEM) t

假设SIZE属于DB.SIZECD。

LEFT JOIN 
(
DB.SIZECD C  
JOIN (SELECT SIZEB FROM DB.LOC WHERE BQTY>0 GROUP BY SIZEB) G ON G.SIZEB = C.SIZE
) ON ITDHGT*ITDWTH*ITDDTH*QTY <= SZCUBC

代替：

LEFT JOIN DB.SIZECD ON ITDHGT*ITDWTH*ITDDTH*QTY <= SZCUBC AND SIZE IN 
(SELECT SIZEB FROM DB.LOC WHERE BQTY>0 GROUP BY SIZEB)

我们无法将带有DB.LOC的子选择移到LEFT JOIN之外以保留逻辑。