Specify the particular database in Java

Generator PDF-417 2d barcode in Java Specify the particular database

Once you re connected to the server, you need to specify which database on the server will be used for your transaction I m using a database called xfd for all the examples in this book The mysql_select_db() function is used to handle this task:

mysql_select_db( xfd );

The reason for connecting to a database is to retrieve data from it (or to add or modify data, but the basic approach is always the same) In any case, you need to pass instructions to the database in SQL (If you re unfamiliar with SQL, it is described in Book VI, 1) Your PHP program usually constructs an SQL statement in a string variable and then passes this value to the database For this basic example, I specify the entire query See the section Processing the input, later in this chapter, for some warnings about how to incorporate user information in data queries The showContactphp program simply asks for a list of all the values in the contact table of the xfd database The SQL query for displaying all the data in a table looks like this:

SELECT * FROM contact;

To use an SQL statement, package it into a string variable, like this:

$sql = SELECT * FROM contact ;

Note that you don t need to include the semicolon inside the string variable You can call the variable anything you wish, but it s commonly called $sql SQL queries can get complex, so if the SQL requires more than one line, you may want to encase it in a heredoc (See 2 of this minibook for information on using heredocs) Pass the request to the database using the msql_query() function:

$result = mysql_query($sql, $conn) or die(mysql_error());

The mysql_query() function has a lot going on Here s how you put it together:

When the query is finished, it will send results back to the program The $result variable will hold that data:

$result = mysql_query($sql, $conn) or die(mysql_error());

2 Invoke mysql_query()

This function passes the query to the database:

$result = mysql_query($sql, $conn) or die(mysql_error());

The first parameter is the query Normally, this is stored in a variable called $sql:

$result = mysql_query($sql, $conn) or die(mysql_error());

The second parameter is the connection object created when you ran mysql_connect() If you leave out the connection object, PHP uses the last MySQL connection that was created:

$result = mysql_query($sql, $conn) or die(mysql_error());

If there s an error in your SQL request, MySQL will send back an error message Prepare for this with the or die() clause (just like you used for mysql_connect()):

$result = mysql_query($sql, $conn) or die(mysql_error());

If something went wrong in the SQL code, have your program reply with the MySQL error so you ll at least know what went wrong:

$result = mysql_query($sql, $conn) or die(mysql_error());

The results of an SQL query are usually data tables, which are a complex data structure The next step when you work with a database is to get all the appropriate information from the $request object and display it in a XHTML output for the user to understand This process is a little involved because SQL results are normally composed of two-dimensional data A query result typically consists of multiple records (information about a specific entity sometimes also called a row) Each record consists of a number of fields (specific data about the current record) I m tossing a bunch of database terms at you here Databases deserve (and have) a minibook of their own If nothing in this chapter makes sense to you, build your own copy of the contact database following the instructions in Book VI and then come back here to have your program present that data through your Web site The $request variable has a lot of data packed into it You get that data out by using a pair of nested loops: