求助数据库中如何求最大值和最小值

sql = "Select 品种名,文件名,参数名,Site_1_平均值,Site_2_平均值,Site_3_平均值,Site_4_平均值,Site_5_平均值,Site_6_平均值,Site_7_平均值,Site_8_平均值 From pingjunzhi Where 日期>=#" & CDate(Combo1.Text) & "# And 日期<= #" & CDate(Combo7.Text) & "# "

If Len(Trim(Combo2.Text)) > 0 Then
sql = sql & " and 品种名 = '" & Trim(Combo2.Text) & "'"
End If

If Len(Trim(Combo3.Text)) > 0 Then
sql = sql & " and 批号 = '" & Trim(Combo3.Text) & "'"
End If

If Len(Trim(Combo4.Text)) > 0 Then
sql = sql & " and 参数名 = '" & Trim(Combo4.Text) & "'"
End If

Adodc2(0).RecordSource = sql

Adodc2(0).Refresh
Set DataGrid2.DataSource = Adodc2(0)