本篇文章给大家介绍11个PHP程序员最常犯的MySQL错误。有一定的参考价值，有需要的朋友可以参考一下，希望对大家有所帮助。
对于大多数web应用来说，数据库都是一个十分基础性的部分。如果你在使用PHP，那么你很可能也在使用MySQL—LAMP系列中举足轻重的一份子。
对于很多新手们来说，使用PHP可以在短短几个小时之内轻松地写出具有特定功能的代码。但是，构建一个稳定可靠的数据库却需要花上一些时日和相关技能。下面列举了我曾经犯过的最严重的11个MySQL相关的错误(有些同样也反映在其他语言/数据库的使用上)。。。
1、使用MyISAM而不是InnoDB
MySQL有很多数据库引擎，但是你最可能碰到的就是MyISAM和InnoDB。
MySQL 默认使用的是MyISAM。但是，很多情况下这都是一个很糟糕的选择，除非你在创建一个非常简单抑或实验性的数据库。外键约束或者事务处理对于数据完整性 是非常重要的，但MyISAM都不支持这些。另外，当有一条记录在插入或者更新时，整个数据表都被锁定了，当使用量增加的时候这会产生非常差的运行效率。
结论很简单：使用InnoDB。
2、使用PHP的mysql函数
PHP自产生之日就提供了MySQL库函数(or near as makes no difference)。很多应用仍然在使用类似mysql_connect、mysql_query、mysql_fetch_assoc等的函数，尽管PHP手册上说：
如果你在使用MySQL v4.1.3或者更新版本，强烈推荐使用您使用mysqli扩展。
mysqli(MySQL的加强版扩展)有以下几个优点：
可选的面向对象接口
prepared表达式，这有利于阻止SQL注入攻击，还能提高性能
支持更多的表达式和事务处理
另外，如果你想支持多种数据库系统，你还可以考虑PDO。
3、没有处理用户输入
这或者可以这样说#1：永远不要相信用户的输入。用服务器端的PHP验证每个字符串，不要寄希望与JavaScript。最简单的SQL注入攻击会利用如下的代码：
$username = $_POST["name"]; $password = $_POST["password"]; $sql = "SELECT userid FROM usertable WHERE username='$username' AND password='$password';"; // run query…
只要在username字段输入“admin';–”，这样就会被黑到，相应的SQL语句如下：
SELECT userid FROM usertable WHERE username='admin';
狡猾的黑客可以以admin登录，他们不需要知道密码，因为密码段被注释掉了。
4、没有使用UTF-8
美国、英国和澳大利亚的我们很少考虑除英语之外的其他语言。我们很得意地完成了自己的“杰作”却发现它们并不能在其他地方正常运行。
UTF-8解决了很多国际化问题。虽然在PHP v6.0之前它还不能很好地被支持，但这并不影响你把MySQL字符集设为UTF-8。
5、相对于SQL，偏爱PHP
如果你接触MySQL不久，那么你会偏向于使用你已经掌握的语言来解决问题，这样会导致写出一些冗余、低效率的代码。比如，你不会使用MySQL自带的AVG()函数，却会先对记录集中的值求和然后用PHP循环来计算平均值。
此外，请注意PHP循环中的SQL查询。通常来说，执行一个查询比在结果中迭代更有效率。
所以，在分析数据的时候请利用数据库系统的优势，懂一些SQL的知识将大有裨益。
6、没有优化数据库查询
99%的PHP性能问题都是由数据库引起的，仅仅一个糟糕的SQL查询就能让你的web应用彻底瘫痪。MySQL的EXPLAIN statement、Query Profiler，还有很多其他的工具将会帮助你找出这些万恶的SELECT。
7、不能正确使用数据类型
MySQL提供了诸如numeric、string和date等的数据类型。如果你想存储一个时间，那么使用DATE或者DATETIME类型。如果这个时候用INTEGER或者STRING类型的话，那么将会使得SQL查询非常复杂，前提是你能使用INTEGER或者STRING来定义那个类型。
很多人倾向于擅自自定义一些数据的格式，比如，使用string来存储序列化的PHP对象。这样的话数据库管理起来可能会变得简单些，但会使得MySQL成为一个糟糕的数据存储而且之后很可能会引起故障。
8、在查询中使用*
永远不要使用*来返回一个数据表所有列的数据。这是懒惰：你应该提取你需要的数据。就算你需要所有字段，你的数据表也不可避免的会产生变化。
9、不使用索引或者过度使用索引
一般性原则是这样的：select语句中的任何一个where子句表示的字段都应该使用索引。
举 个例子，假设我们有一个user表，包括numeric ID(主键)和email address。登录的时候，MySQL必须以一个email为依据查找正确的ID。如果使用了索引的话(这里指email)，那么MySQL就能够使用 更快的搜索算法来定位email，甚至可以说是即时实现。否则，MySQL就只能顺序地检查每一条记录直到找到正确的email address。
有的人会在每个字段上都添加索引，遗憾的是，执行了INSERT或者UPDATE之后这些索引都需要重新生成，这样就会影响性能。所以，只在需要的时候添加索引。
10、忘记备份!
虽然比较罕见，但是数据库还是有崩溃的危险。硬盘有可能损坏，服务器有可能崩溃，web主机提供商有可能会破产!丢失MySQL数据将会是灾难性的，所以请确保你已经使用了自动备份或者已经复制到位。
11、Bonus mistake-不考虑使用其他数据库
对于PHP开发人员来说，MySQL可能是使用最广泛的数据库系统，但并不是唯一的选择。PostgreSQL和Firebird是最强有力的竞争者：这个两者都是开源的，而且都没有被公司收购。微软提供了sql server Express，甲骨文提供了10g Express，这两者都是企业级数据库的免费版本。有时候，对于一个较小的web应用或者嵌入式应用，SQLite也不失为一个可行的替代方案。
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