.NET C#dropdownlist条件查询问题

string mystring = "server=(local);uid=sa;pwd=sa;database=实验人员信息;connect timeout=30";
<br>        SqlConnection myconnection = new SqlConnection(mystring);
<br>        myconnection.Open();
<br>
<br>        string mysql;
<br>        if (ddlsearch.Items[1].Selected)
<br>        {  mysql = "select * from 实验人员 where SY_No=" + "" + this.txtkey.Text + ""; }
<br>        if (ddlsearch.Items[2].Selected)
<br>        {  mysql = "select * from 实验人员 where SY_name=" + "" + this.txtkey.Text + "";}
<br>        if (ddlsearch.Items[3].Selected)
<br>        {  mysql = "select * from 实验人员 where SY_sex=" + "" + this.txtkey.Text + "";}
<br>        if (ddlsearch.Items[4].Selected)
<br>        {  mysql = "select * from 实验人员 where SY_apartment=" + "" + this.txtkey.Text + ""; }
<br>
<br>        
<br>        SqlCommand  mycommand = new SqlCommand(mysql, myconnection);
<br>         mycommand.ExecuteNonQuery();
<br>         myconnection.Close();
<br>         show.DataSource = CreateDataSource();
<br>         show.DataBind();
<br>
<br>编译时出现错误---- "使用了未赋值的局部变量“mysql”"
<br>请高手帮忙解决!!

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