Firstly you should get names of the workbook's sheets, them cycle through these names and do your selects
...
System.Data.DataTable dt = con.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);
if(dt != null){
string[] esheets = new String[dt.Rows.Count];
int i = 0;
foreach(DataRow row in dt.Rows){
esheets[i] = row["TABLE_NAME"].ToString();
i++;
}
for(int j=0; j < esheets.Length; j++){
string query = "Select * from " + esheets[j];
...
}
}
Good luck :)