I suppose that the two tables (Orders and Particulars) are joined by some field that acts as a foreignkey. So you should have a OrderID field in the Particulars table that links every 'particular' to the respective Order.
If this is the case then your query should be something like this
string sqlStatement = @"SELECT Orders.[ID], Orders.[Checkintime],
Orders.[RoomPrice], Orders.[OrderNo],
Particulars.FirstName, Particulars.LastName
FROM Orders INNER JOIN Particulars
ON Orders.[ID] = Particulars.[OrderID]
where Checkintime between '" + dateOnly +
"' and '" + endDateOnly + "'";
However this approach with string concatenation is prone to other kind of errors like parsing problems and Sql injection, better use a parameterized query
string sqlStatement = @"SELECT Orders.[ID], Orders.[Checkintime],
Orders.[RoomPrice], Orders.[OrderNo],
Particulars.FirstName, Particulars.LastName
FROM Orders INNER JOIN Particulars
ON Orders.[ID] = Particulars.[OrderID]
where Checkintime between @init AND @end";
using(SqlConnection cnn = new SqlConnection(.....))
using(SqlCommand cmd = new SqlCommand(sqlStatement, cnn))
{
cnn.Open();
cmd.Parameters.Add("@init", SqlDbType.DateTime).Value = dateOnly;
cmd.Parameters.Add("@end", SqlDbType.DateTime).Value = endDateOnly;
.... remainder of your code that reads back your data.....
}
Please note, the value supplied to the Parameter.Value should be a DateTime variable not a string....