C # with how to determine whether there is a value in the database

A column is selected, such as the user ID column

// user ID to be inserted
String ll_userID = "XXXXXXXX";

// inquiry number exist
SqlCommand mycmd = new SqlCommand ( "select a user ID from table where a user ID = '" + ll_userID + " '", mycon);

the SqlDataReader mysdr mycmd.ExecuteReader = ();

IF (mysdr.HasRows)
  {
     // this number has been recorded using
  }
the else
  {
    // this number is not in use    
  }

mysdr.Close ();
mycon. the Close (); 

---------- If the user ID column is an integer, then, can be written like this

int ll_userID;

// remove the current record number in the maximum value
SqlCommand mycmd = new SqlCommand ( "select max ( user number) from table ", mycon);

the SqlDataReader mysdr mycmd.ExecuteReader = ();

// detect whether there is also recorded the table is to determine whether the object is empty
iF (mysdr.HasRows)
  {
     // table is not empty, the maximum number +1 obtained on the basis of the query, and then insert a new record
  }
the else
  {
    // table is empty, the default insert a minimum number of recording
  }

mysdr.Close ();
mycon.Close ( );