limit the number of register

[router]

Hello!!
I need to limit the number of register that there is in a table. If i want to do an insert and there is already a limit (for example 100), i want that it insert the new data and overwrite the first data (FIFO, first in first out).
For example in t1 there is 100 register..i insert a new data, so it overwirte the 100th register.
How can i limit the number of register in a table?
Any idea? Is it possible?
Thanks in advance
Router

[bastien]

you would have to do a count on the db to see if total records<100..note below code is not tested and would need more to retreive data from whereever you are getting it

//get all the record ids from the table - used (a) to count the num
//of records and (b)to find the smallest id number  available for
//insert/uptdate - use date function to pull earliest date when 
//overwriting the old data

$sql="select id,datestamp from table order by datestamp";
$result=mysql_query($sql); //leave the link stuff up to you
if (!$result){
   echo "there was an error. try again";
}else{
   //see if it is to be insert or update
   $num=mysql_num_rows($result);
   if ($num<100){ 
       //want to insert data
       $sql_insert = "insert into blablabla";
   }esle{
       //want to update the earliest row
       for ($count = 1; $row = mysql_fetch_row ($result); ++$count){
          //assign earliest item to a variable
          $id=$row['id'];

      $sql_update="update tablename set fieldx='valuex', fieldy='valuey' where ID=$id";

      $result_update=mysql_query($sql);  //don't forget link items
      //check if update was successful
      if($result){
          echo "update sucessful!";
      }
       //kill the loop and exit the procedures
       if ($count=2){
          break;
       } 

  }//end for loop

  }//end if loop for insert/update
} //end main if/then loop

?>