Forms Data Controls :: Once Select The Item In The Dropdownlist, It Will Extract Data From Sql Query?

I have a dropdownlist and a button(btnSend).

protected void btnSend_Click(object sender, EventArgs e)
{
InsertIntoTemporary();
LoadAddressdetails();
MassSendOut();
}

What i need to do is as follows;

1) Once i select the item in the dropdownlist, it will extract data from my sql query which is :

string mySQL = "SELECT * FROM examtimetable WHERE subject_name= '" + dd_cat.SelectedValue + "'";

Once seleted,it will display everything in a gridview1.

subject_id subject_name admission_no date venue seat_no