认为他们是dba和数据建模师/设计人员，而他们在这些领域没有任何正式的灌输。 认为他们的项目不需要DBA，因为这些东西都很简单/琐碎。 未能正确区分应该在数据库中完成的工作和应该在应用程序中完成的工作。 未验证备份，或未备份。 在他们的代码中嵌入原始SQL。

忘记在表之间建立关系。我记得当我刚开始在我现在的雇主工作时，我不得不清理这些东西。

我不得不说，应用程序开发人员犯的最大错误是没有正确地规范化数据库。

作为一名应用程序开发人员，我意识到正确的数据库结构、规范化和维护的重要性;我花了无数的时间自学数据库结构和管理。根据我的经验，每当我开始与不同的开发人员合作时，我通常必须重组整个数据库并更新应用程序以适应，因为它通常是畸形的和有缺陷的。

For example, I started working with a new project where the developer asked me to implement Facebook Connect on the site. I cracked open the database to see what I had to work with and saw that every little bit of information about any given user was crammed into one table. It took me six hours to write a script that would organize the table into four or five separate tables and another two to get the app to use those tables. Please, normalize your databases! It will make everything else less of a headache.

没有做正确的标准化。您希望确保数据没有重复，并且根据需要将数据分割为不同的数据。您还需要确保不要过于遵循规范化，否则会损害性能。

不了解DBMS的工作原理。

如果不了解离合器的工作原理，你就不能正确地驾驶变速杆。如果不了解实际上只是在硬盘上写入文件，就无法理解如何使用数据库。

具体地说:

Do you know what a Clustered Index is? Did you think about it when you designed your schema? Do you know how to use indexes properly? How to reuse an index? Do you know what a Covering Index is? So great, you have indexes. How big is 1 row in your index? How big will the index be when you have a lot of data? Will that fit easily into memory? If it won't it's useless as an index. Have you ever used EXPLAIN in MySQL? Great. Now be honest with yourself: Did you understand even half of what you saw? No, you probably didn't. Fix that. Do you understand the Query Cache? Do you know what makes a query un-cachable? Are you using MyISAM? If you NEED full text search, MyISAM's is crap anyway. Use Sphinx. Then switch to Inno.

对于基于sql的数据库:

Not taking advantage of CLUSTERED INDEXES or choosing the wrong column(s) to CLUSTER. Not using a SERIAL (autonumber) datatype as a PRIMARY KEY to join to a FOREIGN KEY (INT) in a parent/child table relationship. Not UPDATING STATISTICS on a table when many records have been INSERTED or DELETED. Not reorganizing (i.e. unloading, droping, re-creating, loading and re-indexing) tables when many rows have been inserted or deleted (some engines physically keep deleted rows in a table with a delete flag.) Not taking advantage of FRAGMENT ON EXPRESSION (if supported) on large tables which have high transaction rates. Choosing the wrong datatype for a column! Not choosing a proper column name. Not adding new columns at the end of the table. Not creating proper indexes to support frequently used queries. creating indexes on columns with few possible values and creating unnecessary indexes. ...more to be added.