此备份脚本中的“drop table”和“create table”选项有什么作用？

I need a script to back up a MYSQL database that I don't have cpanel, shell, or phpmyadmin access to.

I'm just concerned about the DROP TABLE part of this script and why it would be needed. I do not want to modify the database at all, I just want a backup.

Here is my code:

backup_tables('localhost','username','password','blog');


/* backup the db OR just a table */
function backup_tables($host,$user,$pass,$name,$tables = '*')
{

$link = mysql_connect($host,$user,$pass);
mysql_select_db($name,$link);

//get all of the tables
if($tables == '*')
{
    $tables = array();
    $result = mysql_query('SHOW TABLES');
    while($row = mysql_fetch_row($result))
    {
        $tables[] = $row[0];
    }
}
else
{
    $tables = is_array($tables) ? $tables : explode(',',$tables);
}

//cycle through
foreach($tables as $table)
{
    $result = mysql_query('SELECT * FROM '.$table);
    $num_fields = mysql_num_fields($result);

    $return.= 'DROP TABLE '.$table.';';
    $row2 = mysql_fetch_row(mysql_query('SHOW CREATE TABLE '.$table));
    $return.= "

".$row2[1].";

";

    for ($i = 0; $i < $num_fields; $i++) 
    {
        while($row = mysql_fetch_row($result))
        {
            $return.= 'INSERT INTO '.$table.' VALUES(';
            for($j=0; $j<$num_fields; $j++) 
            {
                $row[$j] = addslashes($row[$j]);
                $row[$j] = ereg_replace("
","\
",$row[$j]);
                if (isset($row[$j])) { $return.= '"'.$row[$j].'"' ; } else { $return.= '""'; }
                if ($j<($num_fields-1)) { $return.= ','; }
            }
            $return.= ");
";
        }
    }
    $return.="


";
}

//save file
$handle = fopen('db-backup-'.time().'-'.(md5(implode(',',$tables))).'.sql','w+');
fwrite($handle,$return);
fclose($handle);
}

So this script is generating a file with all of the SQL commands to recreate the current database.

So once the script is done executing you could "import" the file it creates in phpmyadmin and it will drop the tables if they exist and then insert all of the data the was in the tables at the time of the backup.

It will not modify anything from the current database

For example, this is what the "export" function in phpmyadmin will create for a test table:

--
-- Database: `test`
--

-- --------------------------------------------------------

--
-- Table structure for table `test_table`
--

DROP TABLE IF EXISTS `test_table`;
CREATE TABLE IF NOT EXISTS `test_table` (
  `id` int(11) NOT NULL,
  `num` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

--
-- Dumping data for table `test_table`
--

INSERT INTO `test_table` (`id`, `num`) VALUES
(1, 23),
(2, 45);

Just a serious of SQL statements to recreate a database.