主题：[求助]组合多个统计结果,问，要按日期的月份统计，应该怎么写？

Dim bd1 As New GroupTableBuilder("统计表1",DataTables("进货单"))
Dim dt1 As fxDataSource
bd1.Groups.AddDef("型号") '根据型号分组
bd1.Totals.AddDef("数量","进货_数量") '对数量进行统计
bd1.Totals.AddDef("金额","进货_金额") '对金额进行统计
dt1 = bd1.BuildDataSource()

Dim bd2 As New GroupTableBuilder("统计表2",DataTables("销售单"))
Dim dt2  As fxDataSource
bd2.Groups.AddDef("型号") '根据型号分组
bd2.Totals.AddDef("数量","销售_数量") '对数量进行统计
bd2.Totals.AddDef("金额","销售_金额") '对金额进行统计
dt2 = bd2.BuildDataSource()

Dim bd3 As New GroupTableBuilder("统计表3",DataTables("退货单"))
Dim dt3 As fxDataSource
bd3.Groups.AddDef("型号") '根据型号分组
bd3.Totals.AddDef("数量","退货_数量") '对数量进行统计
bd3.Totals.AddDef("金额","退货_金额") '对金额进行统计
dt3 = bd3.BuildDataSource()

dt1.Combine("型号",dt2,"型号") '将销售统计数据组合到进货统计数据
dt1.Combine("型号",dt3,"型号") '将退货统计数据组合到进货统计数据

Tables("窗口1_Table1").DataSource = dt1 '将统计结果绑定到Table
With DataTables("窗口1_Table1").DataCols  '用表达式列计算库存数据
    .Add("库存_数量",Gettype(Integer), "IsNull([进货_数量],0) - ISNULL([销售_数量],0) - ISNULL([退货_数量],0)")
    .Add("库存_金额",Gettype(Double), "[库存_数量] /[进货_数量] * [进货_金额]") 
End With