$db1 = mysql_connect("hostname","username","password") // Connect to Server 1
  mysql_select_db("db1″, $db1);
  $res1 = mysql_query("query",$db1);

  $db2 = mysql_connect("hostname","username","password"); // Connect to Server 2
  mysql_select_db("db2″, $db2);
  $res2 = mysql_query("query",$db2);

At this point you can only fetch records from your previous ResultSet, i.e $res1 – But you cannot execute new query in $db1, even if you supply the link as the link was overwritten by the new db, So at this point the following script will be failed.

$res3 = mysql_query("query",$db1); //this will failSo how to solve that?

Please have a look at.

	$db1 = mysql_connect("hostname","username","password");
	mysql_select_db("db1″, $db1);
	$res1 = mysql_query("query",$db1);

	$db2 = mysql_connect("hostname","username","password", true)
	mysql_select_db("db2", $db2);
	$res2 = mysql_query("query",$db2);

So mysql_connect has another optional boolean parameter which indicates whether a link will be created or not.

As we connect to the $db2 with this optional parameter set to ‘true’, so both link will remain live.

Now the following query will execute successfully.

	$res3 = mysql_query("query",$db1);

DOUBLEs store floating point numbers with 16 place accuracy and take up 8 bytes.

REAL is a synonym of FLOAT for now.

2. Rename Postcode.txt to Postcode.sql

3. Import to your database and enjoy complete list of UK Postcodes.

Here is the sample code:

CREATE TABLE IF NOT EXISTS `postcodes` (
  `id` int(11) NOT NULL auto_increment,
  `postcode` tinytext NOT NULL,
  `grid_n` varchar(15) NOT NULL,
  `grid_e` varchar(15) NOT NULL,
  `latitude` varchar(15) NOT NULL,
  `longitude` varchar(15) NOT NULL,
  PRIMARY KEY  (`id`)
);

INSERT INTO `postcodes` (`id`, `postcode`, `grid_n`, `grid_e`, `latitude`, `longitude`) VALUES
(1, 'AB10', '392900', '804900', '57.135', '-2.117'),
(2, 'AB11', '394500', '805300', '57.138', '-2.092'),
(3, 'AB12', '393300', '801100', '57.101', '-2.111'),
(4, 'AB13', '385600', '801900', '57.108', '-2.237'),
(5, 'AB14', '383600', '801100', '57.101', '-2.27'),
(6, 'AB15', '390000', '805300', '57.138', '-2.164');

mysql_connect() and mysql_pconnect() both are working for database connection but with small difference. In mysql_pconnect(), ‘p’ stands for persistence connection.

But in case of mysql_pconnect() function,
first, when connecting, the function would try to find a (persistent) connection that’s already open with the same host, username and password. If three is found, an identifier for it will be returned in lieu of opening a new connection.
Second, the connection to the SQL server won’t be closed when the execution of the script ends. In lieu, the connection will remain open for future use (mysql_close() won’t close connection established by mysql_pconnect()).

Mysql_pconncet() is useful when you have a lot of traffic on your site. At that time for every request it won’t open a connection but will take it from the pool. This will increase the efficiency of your site. But for general use mysql_connect() is best.

Indexes should be used to increase the performance of MySQl select query. Index on a column can be specified using col_name(n) syntax. Here, n specifies the first “n” characters that can be used to create an index. Columns that will be used more often in a select query should be indexed. When a new index is created; MYSQl builds a separate block of information that needs to be updated every time there are changes made to the table.

MySQL optimizes the following way:

• Removes unnecessary parenthesis
• Constant expressions used by indexes are evaluated only once
• Early detection of invalid constant expressions
• HAVING is merged with WHERE if GROUPBY is not used.
• The best Join combination is found.
• Before each row is output, those that do not match the HAVING clause are skipped.
• Each table index is queried and the best index is used.

MyISAM is the default storage engine.

MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the MyISAM storage engine in MySQL 3.23, the maximum table size was increased to
65536 terabytes (2567 – 1 bytes). With this larger allowed table size, the maximum effective table size for MySQL databases is usually determined by operating system constraints on file sizes, not by MySQL internal limits.The InnoDB storage engine maintains InnoDB tables within a tablespace
that can be created from several files. This allows a table to exceed the maximum individual file size. The tablespace can include raw disk
partitions, which allows extremely large tables. The maximum tablespace size is 64TB. The following table lists some examples of operating system file-size limits. This is only a rough guide and is not intended to be definitive. For the most up-to-date information, be sure to check the documentation specific to your operating system.

Operating System File-size LimitLinux 2.2-Intel 32-bit 2GB (LFS: 4GB)

Linux 2.4+ (using ext3 filesystem) 4TB

Solaris 9/10 16TB

NetWare w/NSS filesystem 8TB

Win32 w/ FAT/FAT32 2GB/4GB

Win32 w/ NTFS 2TB (possibly larger)

MacOS X w/ HFS+ 2TB

• The ‘.frm’ file stores the table definition.
• The data file has a ‘.MYD’ (MYData) extension.
• The index file has a ‘.MYI’ (MYIndex) extension.