5 Easy-to-commit PHP database errors---Include database schema design, database access, and business logic code that uses the database---and their solutions.
If only one way to use the database is correct ...
You can create database design, database access, and database-based PHP business logic code in a number of ways, but in the end it usually ends in error. This article explains the five common problems that occur in the database design and the PHP code that accesses the database, and how to fix them when they encounter these problems.
Issue 1: Using MySQL directly
A common problem is that older PHP code uses the MYSQL_ function directly to access the database. Listing 1 shows how to access the database directly.
Listing 1. access/get.php
<?php
function get_user_id ($name)
{
$db = mysql_connect (' localhost ', ' root ', ' PassWord ');
mysql_select_db (' users ');
$res = mysql_query ("Select ID from Users WHERE login= '". $name. "");
while ($row = Mysql_fetch_array ($res)) {$id = $row [0];}
return $id;
}
Var_dump (get_user_id (' Jack '));
? >
Note The Mysql_connect function is used to access the database. Also note the query, which uses a string connection to add $name parameters to the query.
There are two good alternatives to this technique: the PEAR DB module and the PHP Data Objects (PDO) class. Both provide abstraction from a specific database selection. So, your code doesn't need to be too much tuned for IBM? Run on DB2, MySQL, PostgreSQL, or any other database you want to connect to.
Another value of using the PEAR DB module and the PDO abstraction layer is that you can use it in SQL statements. Operator. Doing so makes SQL easier to maintain and can protect your application from SQL injection attacks.
The alternative code that uses PEAR DB is shown below.
Listing 2. access/get_good.php
<?php
Require_once ("db.php");
function get_user_id ($name)
{
$dsn = ' mysql://root:password@localhost/users ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$res = $db->query (' SELECT ID from users WHERE login=? ', Array ($name));
$id = null;
while ($res->fetchinto ($row)) {$id = $row [0];}
return $id;
}
Var_dump (get_user_id (' Jack '));
? >
Note that all the places that are used directly to MySQL are eliminated, except for the database connection string in the $dsn. In addition, we pass? operator to use $name variable in SQL. The queried data is then sent in through an array at the end of the query () method.
Issue 2: Do not use the auto-increment feature
Like most modern databases, MySQL can create an AutoIncrement unique identifier on a per-record basis. In addition, we will still see the code that runs a SELECT statement first to find the maximum ID, then adds 1 to the ID and finds a new record. Listing 3 shows an example of a bad pattern.
Listing 3. Badid.sql
DROP TABLE IF EXISTS users;
CREATE TABLE Users (
ID Mediumint,
Login TEXT,
Password TEXT
);
INSERT into Users VALUES (1, ' Jack ', ' Pass ');
INSERT into Users VALUES (2, ' Joan ', ' Pass ');
INSERT into Users VALUES (1, ' Jane ', ' Pass ');
The ID field here is simply specified as an integer. So, although it should be unique, we can add any value, as shown in several INSERT statements following the CREATE statement. Listing 4 shows the PHP code that adds the user to this type of pattern.
Listing 4. add_user.php
<?php
Require_once ("db.php");
function Add_user ($name, $pass)
{
$rows = Array ();
$dsn = ' Mysql://root:password@localhost/bad_badid ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$res = $db->query ("SELECT Max (ID) from users");
$id = null;
while ($res->fetchinto ($row)) {$id = $row [0];}
$id + = 1;
$sth = $db->prepare ("INSERT into Users VALUES (?,?,?)");
$db->execute ($sth, Array ($id, $name, $pass));
return $id;
}
$id = Add_user (' Jerry ', ' Pass ');
Var_dump ($id);
? >
The code in add_user.php first executes a query to find the maximum value of the ID. The file then runs an INSERT statement with the ID value plus 1. The code will fail in a race condition on a heavily loaded server. In addition, it is inefficient.
So what is the alternative? Use the auto-increment feature in MySQL to automatically create a unique ID for each insert. The updated pattern is shown below.
Listing 5. goodid.php
DROP TABLE IF EXISTS users;
CREATE TABLE Users (
ID mediumint not NULL auto_increment,
Login TEXT not NULL,
Password TEXT not NULL,
PRIMARY KEY (ID)
);
INSERT into users VALUES (null, ' Jack ', ' Pass ');
INSERT into users VALUES (null, ' Joan ', ' Pass ');
INSERT into users VALUES (null, ' Jane ', ' Pass ');
We have added a NOT NULL flag to indicate that the field must not be empty. We also added the AUTO_INCREMENT flag to indicate that the field is AutoIncrement, adding the PRIMARY KEY flag to indicate that the field is an ID. These changes speed up. Listing 6 shows the updated PHP code, which will insert the user into the table.
Listing 6. add_user_good.php
<?php
Require_once ("db.php");
function Add_user ($name, $pass)
{
$dsn = ' Mysql://root:password@localhost/good_genid ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$sth = $db->prepare ("INSERT into users VALUES (null,?,?)");
$db->execute ($sth, Array ($name, $pass));
$res = $db->query ("Select last_insert_id ()");
$id = null;
while ($res->fetchinto ($row)) {$id = $row [0];}
return $id;
}
$id = Add_user (' Jerry ', ' Pass ');
Var_dump ($id);
? >
Now that I'm not getting the maximum ID value, I'm using the INSERT statement to insert the data, and then use the SELECT statement to retrieve the ID of the last inserted record. The code is much simpler and more efficient than the original version and its associated schema.
Issue 3: Using multiple databases
Occasionally, we see an application in which each table is in a separate database. It is reasonable to do so in a very large database, but for a generic application, this level of fragmentation is not required. In addition, relational queries cannot be executed across databases, which can affect the overall idea of using relational databases, not to mention the difficulty of managing tables across multiple databases. So, what should multiple databases look like? First, you need some data. Listing 7 shows such data divided into 4 files.
Listing 7. Database files
Files.sql:
CREATE TABLE Files (
ID Mediumint,
USER_ID Mediumint,
Name TEXT,
Path TEXT
);
Load_files.sql:
INSERT into Files VALUES (1, 1, ' test1.jpg ', ' files/test1.jpg ');
INSERT into Files VALUES (2, 1, ' test2.jpg ', ' files/test2.jpg ');
Users.sql:
DROP TABLE IF EXISTS users;
CREATE TABLE Users (
ID Mediumint,
Login TEXT,
Password TEXT
);
Load_users.sql:
INSERT into Users VALUES (1, ' Jack ', ' Pass ');
INSERT into Users VALUES (2, ' Jon ', ' Pass ');
In the multi-database version of these files, you should load the SQL statements into one database, and then load the users SQL statements into another database. The PHP code used to query the database for files associated with a particular user is shown below.
Listing 8. getfiles.php
<?php
Require_once ("db.php");
function Get_user ($name)
{
$dsn = ' mysql://root:password@localhost/bad_multi1 ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$res = $db->query ("Select ID from Users WHERE login=?", Array ($name));
$uid = null;
while ($res->fetchinto ($row)) {$uid = $row [0];}
return $uid;
}
function Get_files ($name)
{
$uid = Get_user ($name);
$rows = Array ();
$dsn = ' Mysql://root:password@localhost/bad_multi2 ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$res = $db->query ("select * from Files WHERE user_id=?", Array ($uid));
while ($res->fetchinto ($row)) {$rows [] = $row;}
return $rows;
}
$files = Get_files (' Jack ');
Var_dump ($files);
? >
The Get_user function connects to the database that contains the user table and retrieves the ID of the given user. The Get_files function connects to a file table and retrieves the file rows associated with a given user.
A better way to do all of these things is to load the data into a database and then execute the query, such as the following query.
Listing 9. getfiles_good.php
<?php
Require_once ("db.php");
function Get_files ($name)
{
$rows = Array ();
$dsn = ' Mysql://root:password@localhost/good_multi ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$res = $db->query ("Select files.* from users, files WHERE
Users.login=? and users.id=files.user_id ",
Array ($name));
while ($res->fetchinto ($row)) {$rows [] = $row;}
return $rows;
}
$files = Get_files (' Jack ');
Var_dump ($files);
? >
The code is not only shorter, but also easier to understand and efficient. Instead of executing two queries, we execute a query.
Although the problem may sound far-fetched, in practice we usually conclude that all tables should be in the same database, unless there is a compelling reason to do so. Issue 4: Do not use relationships
Relational databases are different from programming languages, and they do not have an array type. Instead, they use relationships between tables to create one-to-many structures between objects, which have the same effect as arrays. One of the problems I see in the application is that the engineer is trying to use the database as a programming language to create an array by using a text string with a comma-delimited identifier. Take a look at the pattern below.
Listing 10. Bad.sql
DROP TABLE IF EXISTS files;
CREATE TABLE Files (
ID Mediumint,
Name TEXT,
Path TEXT
);
DROP TABLE IF EXISTS users;
CREATE TABLE Users (
ID Mediumint,
Login TEXT,
Password TEXT,
Files TEXT
);
INSERT into Files VALUES (1, ' test1.jpg ', ' media/test1.jpg ');
INSERT into Files VALUES (2, ' test1.jpg ', ' media/test1.jpg ');
INSERT into Users VALUES (1, ' Jack ', ' pass ', ' + ');
A user in the system can have multiple files. In programming languages, you should use arrays to represent the files associated with a user. In this example, the programmer chooses to create a files field that contains a comma-delimited list of file IDs. To get a list of all the files for a particular user, the programmer must first read the rows from the user table, parse the text of the file, and run a separate SELECT statement for each file. The code is shown below.
Listing 11. get.php
<?php
Require_once ("db.php");
function Get_files ($name)
{
$dsn = ' Mysql://root:password@localhost/bad_norel ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$res = $db->query ("Select files from users WHERE login=?", Array ($name));
$files = null;
while ($res->fetchinto ($row)) {$files = $row [0];}
$rows = Array ();
foreach (Split (', ', $files) as $file)
{
$res = $db->query ("select * from Files WHERE id=?",
Array ($file));
while ($res->fetchinto ($row)) {$rows [] = $row;}
}
return $rows;
}
$files = Get_files (' Jack ');
Var_dump ($files);
? >
The technology is slow, difficult to maintain, and does not make good use of the database. The only solution is to re-architect the schema to convert it back to the traditional relational form, as shown below.
Listing 12. Good.sql
DROP TABLE IF EXISTS files;
CREATE TABLE Files (
ID Mediumint,
USER_ID Mediumint,
Name TEXT,
Path TEXT
);
DROP TABLE IF EXISTS users;
CREATE TABLE Users (
ID Mediumint,
Login TEXT,
Password TEXT
);
INSERT into Users VALUES (1, ' Jack ', ' Pass ');
INSERT into Files VALUES (1, 1, ' test1.jpg ', ' media/test1.jpg ');
INSERT into Files VALUES (2, 1, ' test1.jpg ', ' media/test1.jpg ');
Here, each file is related to the user in the file table through the USER_ID function. This may be contrary to the thought of anyone who sees multiple files as arrays. Of course, the array does not refer to its contained objects-in fact, vice versa. But in a relational database, that's how it works, and queries are fast and much simpler. Listing 13 shows the corresponding PHP code.
Listing 13. get_good.php
<?php
Require_once ("db.php");
function Get_files ($name)
{
$dsn = ' Mysql://root:password@localhost/good_rel ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
$rows = Array ();
$res = $db->query ("Select files.* from Users,files WHERE users.login=?
and users.id=files.user_id ", Array ($name));
while ($res->fetchinto ($row)) {$rows [] = $row;}
return $rows;
}
$files = Get_files (' Jack ');
Var_dump ($files);
? >
Here, we make a query to the database to get all the rows. The code is not complex, and it uses the database as its original purpose.
Problem 5:n+1 Mode
I don't know how many times I've seen such a large application, where the code first retrieves some entities (such as customers) and then retrieves them back and forth to get the details of each entity. We call it the n+1 pattern because the query executes so many times-one query retrieves a list of all entities, and then executes the query for each of the N entities. This is not a problem when n=10, but when n=100 or n=1000? Then there is a certain inefficiency problem. Listing 14 shows an example of this pattern.
Listing 14. Schema.sql
DROP TABLE IF EXISTS authors;
CREATE TABLE Authors (
ID mediumint not NULL auto_increment,
Name TEXT not NULL,
PRIMARY KEY (ID)
);
DROP TABLE IF EXISTS Books;
CREATE TABLE Books (
ID mediumint not NULL auto_increment,
author_id Mediumint not NULL,
Name TEXT not NULL,
PRIMARY KEY (ID)
);
INSERT into authors VALUES (null, ' Jack Herrington ');
INSERT into authors VALUES (null, ' Dave Thomas ');
INSERT into books VALUES (null, 1, ' Code Generation in Action ');
INSERT into books VALUES (null, 1, ' podcasting Hacks ');
INSERT into books VALUES (null, 1, ' PHP Hacks ');
INSERT into books VALUES (null, 2, ' pragmatic Programmer ');
INSERT into books VALUES (null, 2, ' Ruby on Rails ');
INSERT into books VALUES (null, 2, ' programming Ruby ');
The pattern is reliable and there are no errors. The problem is in the code that accesses the database to find all the books for a given author, as shown below.
Listing 15. get.php
<?php
Require_once (' db.php ');
$dsn = ' mysql://root:password@localhost/good_books ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
function get_author_id ($name)
{
Global $db;
$res = $db->query ("SELECT ID from authors WHERE name=?", Array ($name));
$id = null;
while ($res->fetchinto ($row)) {$id = $row [0];}
return $id;
}
function Get_books ($id)
{
Global $db;
$res = $db->query ("SELECT ID from books WHERE author_id=?", Array ($id));
$ids = Array ();
while ($res->fetchinto ($row)) {$ids []= $row [0];}
return $ids;
}
function Get_book ($id)
{
Global $db;
$res = $db->query ("select * from books WHERE id=?", Array ($id));
while ($res->fetchinto ($row)) {return $row;}
return null;
}
$author _id = get_author_id (' Jack Herrington ');
$books = Get_books ($author _id);
foreach ($books as $book _id) {
$book = Get_book ($book _id);
Var_dump ($book);
}
? >
If you look at the following code, you might think, "Hey, that's really clear." "First, get the author ID, then get a list of books and get information about each book." Yes, it's clear, but is it efficient? The answer is in the negative. See how many queries are executed when you retrieve only Jack Herrington's books. Get the ID one at a time, get a list of books for another time, and then execute the query once for each book. Three books to execute five queries!
The solution is to use a function to execute a large number of queries, as shown below.
Listing 16. get_good.php
<?php
Require_once (' db.php ');
$dsn = ' mysql://root:password@localhost/good_books ';
$db =& db::connect ($DSN, Array ());
if (Pear::iserror ($db)) {die ($db->getmessage ());}
function Get_books ($name)
{
Global $db;
$res = $db->query ("Select books.* from Authors,books WHERE books.author_id=authors.id and authors.name=?",
Array ($name));
$rows = Array ();
while ($res->fetchinto ($row)) {$rows []= $row;}
return $rows;
}
$books = Get_books (' Jack Herrington ');
Var_dump ($books);
? >
Retrieving the list now requires a quick, single query. This means that I would most likely have to have several of these types of methods with different parameters, but there is no choice. If you want to have an extended PHP application, you must use the database effectively, which means more intelligent queries.
The problem with this example is that it's a little too clear. Generally, these types of n+1 or n*n problems are much more subtle. And they only appear when the database administrator runs the query Profiler on the system when the system has performance problems.
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