我无法修复以下错误：
`“您的 SQL 语法有错误；请查看与您的 MySQL 服务器版本相对应的手册，了解在 'When,Then) 值附近使用的正确语法（'79','WBT-CoE','gyj',' yi','yi')' 在第 1 行"` 错误。
这是代码：
protected void Button3_Click(object sender, EventArgs e){
string MyconnectionString = "server=localhost;database=requirement_doc;Uid=t;Pwd=123;";
MySqlConnection conn = new MySqlConnection(MyconnectionString);
MySqlCommand cmd;
DataTable dt1 = new DataTable();
cmd = conn.CreateCommand();
cmd.CommandText = "SELECT Req_ID, Actor FROM UseCase where Req_ID='" + txtReqID.Text + "' AND Actor='" + DropDownList1.Text + "'";
MySqlDataAdapter da1 = new MySqlDataAdapter();
da1.SelectCommand = cmd;
da1.Fill(dt1);
if (dt1.Rows.Count > 0)
{
Label1.Text = "Data already exist";
}
else
{
string sql = "INSERT INTO UseCase (Req_ID,Actor,Given,When,Then) values( '" + txtReqID.Text + "','" + DropDownList1.Text + "','" + giventxt.Text + "','" + whentbl.Text + "','" + thentbl.Text + "')";
cmd.Connection = conn;
cmd.CommandText = sql;
conn.Open();
}
try
{
cmd.ExecuteNonQuery();
Label1.Text = " Successfully saved";
}
catch (Exception ex)
{
throw new Exception(ex.Message);
}
}
}