Private Sub Command1_Click()
Dim aa As Integer
 txtsql = ""
If Trim(Text1(0).Text) <> "" Then
  aa = 1
  If txtsql = "" Then                                      这边的txtsql是什么作用，有定义全局变量，帮忙看看
    txtsql = "xj.学号='" & Trim(Text1(0).Text) & "'"
  Else
    txtsql = txtsql & "and xj.学号='" & Trim(Text1(0).Text) & "'"
  End If
End If
If Trim(Text1(1).Text) <> "" Then
aa = 1                                                      这边的aa是什么作用，也帮忙看看
  If txtsql = "" Then
    txtsql = "xj.姓名='" & Trim(Text1(1).Text) & "'"
  Else
    txtsql = txtsql & "and xj.姓名='" & Trim(Text1(1).Text) & "'"
  End If
End If
If Trim(Text1(2).Text) <> "" Then
  aa = 1
  If txtsql = "" Then
    txtsql = "xj.班级='" & Trim(Text1(2).Text) & "'"
  Else
    txtsql = txtsql & "and xj.班级='" & Trim(Text1(2).Text) & "'"
  End If
End If
If Trim(Text1(3).Text) <> "" Then
 aa = 1
 txtsql = ""
txtsql = "xj.姓名 like '%" & Trim(Text1(3).Text) & "%'"

End If
If aa = 0 Then
  ss = MsgBox("你至少要输入一个条件以上才能查询！", , " 警告")
  Text1(0).SetFocus
  Exit Sub
  
End If

aa  标记 是否有条件查询,只要有一个条件,那么 aa=1  ,否则 aa=0
依次在各个条件段中进行赋值.

txtsql 是组合的查询条件变量

If Trim(Text1(0).Text) <> "" Then                                          '内容不为空
  aa = 1                       '设置有条件
  If txtsql = "" Then                       '原查询条件为空               
    txtsql = "xj.学号='" & Trim(Text1(0).Text) & "'"        '直接生成查询条件
  Else '否则
    txtsql = txtsql & "and xj.学号='" & Trim(Text1(0).Text) & "'"       '加上面的条件,再生成查询条件
  End If
End If

高手，一语惊醒，谢谢啊