2.Where-used (Top Where-used) / 用途（父件产品及顶层产品）

    Where-used一般用于查询物料的用途，即在哪些BOM中用到了该物料。而Top Where-used则用于查询该物料所用于的最终产品信息。在制造业中，这些都是非常有用的资料，比如在一个ECN中需要更改一个物料，那么通过Top Where-used即可以马上知道这个ECN会影响到哪些产品。

    在Where-used的算法中，关键是是否要考虑BOM的版本，以及生效/失效日期等。

    以下是一个经典的Where-used (Top Where-used) 的示例程序。其中用到了Container来储存信息，其实用数据表(Table) 也是同样可以的。

void Whereused(sourceItemId, bomDate = today())
{
    BOM                   BOM_A, BOM_B;
    BOMVersion       bomVersion;
    Set                     tmpContainer, tmpWhereusedItems, tmpWhereusedTopItems;
    SetEnumerator  se;
    ItemId               tmpItemId;
    ;

    // Initialization
    tmpContainer                   = new Set(Types::String);
    tmpWhereusedItems       = new Set(Types::String);
    tmpWhereusedTopItems = new Set(Types::String);

    // Get all the active boms
    while select BOM_A
           where BOM_A.ItemId == sourceItemId
              && (!BOM_A.ToDate   || BOM_A.ToDate   >= bomDate)
              && (!BOM_A.FromDate || BOM_A.FromDate <= bomDate)
    {
        select firstonly bomVersion
                     where bomVersion.BOMId    == BOM_A.BOMId
                           && bomVersion.Active   == true
                           && bomVersion.Approved == true
                           && (!bomVersion.FromDate || bomVersion.FromDate <= bomDate)
                           && (!bomVersion.ToDate   || bomVersion.ToDate   >= bomDate);

        if (bomVersion
            && (!tmpWhereusedItems.in(bomVersion.ItemId)
            && !tmpContainer.in(bomVersion.ItemId)))
        {
            tmpContainer.add(bomVersion.ItemId);
        }
    }

    //loop the program while the temporary container have ItemId
    se = tmpContainer.getEnumerator();
    while (!tmpContainer.empty())
    {
        // get the first one
        se.moveNext();
        tmpItemId = se.current();
        // remove -> reduce the element count
        tmpContainer.remove(tmpItemId);
        tmpWhereusedItems.add(tmpItemId);
        se.reset();

        select firstonly BOM_B
                   where BOM_B.ItemId == tmpItemId
                         && (!BOM_B.ToDate   || BOM_B.ToDate   >= bomDate)
                         && (!BOM_B.FromDate || BOM_B.FromDate <= bomDate);

        if (BOM_B) //if the tmpItemId is still used in other boms
        {
            //get all ItemIds which consists the tmpItemId
            while select BOM_B
                   where BOM_B.ItemId == tmpItemId
                         && (!BOM_B.ToDate   || BOM_B.ToDate   >= bomDate)
                         && (!BOM_B.FromDate || BOM_B.FromDate <= bomDate)
            {
                select firstonly bomVersion
                             where bomVersion.BOMId    == BOM_B.BOMId
                                   && bomVersion.Active   == true
                                   && bomVersion.Approved == true
                                   && (!bomVersion.FromDate || bomVersion.FromDate <= bomDate)
                                   && (!bomVersion.ToDate   || bomVersion.ToDate   >= bomDate);

                if (bomVersion
                    && (!tmpWhereusedItems.in(bomVersion.ItemId)
                    && !tmpContainer.in(bomVersion.ItemId)))
                {
                    // for SET, there will ensure no duplication
                    tmpContainer.add(bomVersion.ItemId);
                }
            }
        }
        else // if the tmpItemId is not used in other boms, this is the topest layer ItemId
        {
            select firstonly bomVersion
                         where bomVersion.ItemId   == tmpItemId
                               && bomVersion.Active   == true
                               && bomVersion.Approved == true
                               && (!bomVersion.FromDate || bomVersion.FromDate <= bomDate)
                               && (!bomVersion.ToDate   || bomVersion.ToDate   >= bomDate);

            if (bomVersion)
            {
                // get the ItemId, and put it to another container
                {
                    tmpWhereusedTopItems.add(tmpItemId);
                }
            }
        }
    }
}