PHP

Locking with an auxiliary table

If values must be shown to a user, consider adding a summary table for identifiers, or copying rows to a temporary table. For example, an identifier table can store the next available identifier for each other table, this can then be incremented by the script and the value can be used in subsequent scripts without locking problems and without any clashes in numbering.

This solution is shown in Example 6-10, using an auxiliary table named ids that manages the next available region_id attribute. The use of the additional table prevents duplicate rows being inserted, and avoids any problems with locking or updates.

Example 6-10. An auxiliary table manages the next region_id attribute
<?php
  // This code needs an auxiliary table called "ids"
  // that might be created with:
  // CREATE TABLE ids (
  // region_id int default 0,
  // other_id int default 0,
  // another_id int default 0
  // );
  // It has one row, and no primary key is required.
  // After creating the table, a row is needed,
  // so issue an: INSERT INTO ids (NULL, NULL, NULL);
  // (if it's being added later, use MAX(  ) to get the
  //  correct ID values!)
  include 'db.inc';
  include 'error.inc';
  function getNextRegion ($connection)
  {
     // A nice way to do it... use an auxiliary table
     // Lock the auxiliary table
     $query = "LOCK TABLES ids WRITE";
     if (!mysql_query($query, $connection))
        showerror(  );
     // Add one to the region_id attribute
     $query = "UPDATE ids SET region_id = region_id + 1";
     if (!mysql_query($query, $connection))
        showerror(  );
     // Find out the new value of region_id
     $query = "SELECT * FROM ids";
     if (!($result = mysql_query($query, $connection)))
        showerror(  );
     // Get the row that is returned
     $row = mysql_fetch_array($result);
     // Unlock the table
     $query = "UNLOCK TABLES";
     if (!mysql_query($query, $connection))
        showerror(  );
     // Return the region_id
     return ($row["region_id"]);
  }
  // MAIN -----
  if (!($connection = @ mysql_connect($hostName,
                                    $username,
                                    $password)))
     die("Could not connect to database");
  if (!mysql_select_db($databaseName))
     showerror(  );
  if (empty($regionId))
  {
     $regionId =
        getNextRegion($connection, $databaseName);
?>
<!DOCTYPE HTML PUBLIC
   "-//W3C//DTD HTML 4.0 Transitional//EN"
   "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
  <title>Insert a region</title>
</head>
<body bgcolor="white">
region_id: <?= $regionId ?>
<br>
<form method="post" action="example.6-10.php">
  <input type="hidden"
   name="regionId" value="<?=$regionId;?>">
  <br>region_name:
  <br><input type="text" name="regionName" size=80>
  <br>description:
  <br><textarea name="description" rows=4 cols=80>
      </textarea>
  <br><input type="submit">
</form>
</body>
</html>
<?php
  }
  else
  {
     $regionId = clean($regionId, 3);
     $regionName = clean($regionName, 20);
     $description = clean($description, 255);
     $query = "INSERT INTO region SET " .
       "region_id = " . $regionId . ", " .
       "region_name = \"" . $regionName . "\", " .
       "description = \"" . $description . "\"";
     if ((@ mysql_query ($query, $connection))
             && @ mysql_affected_rows(  ) == 1)
        header("Location:insert_receipt.php?" .
               "values=$regionId&status=T");
     else
        header("Location: insert_receipt.php?status=F");
  }
?>

by BrainBellupdated
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