有一张桌子

code_1
value_1
code_2
value_2
...
code_10
value_10

而不是有3个表

Code, value和code_value

你永远不知道什么时候你可能需要10对以上的代码，价值。

如果只需要一对，就不会浪费磁盘空间。

FROM TableA, TableB WHERE语法用于连接而不是FROM TableA内部连接TableB上 假设查询将以某种方式返回，而不放入ORDER BY子句，因为这是在查询工具中测试时显示的方式。

编写查询的开发人员没有很好地了解SQL应用程序(包括单个查询和多用户系统)的快慢。这包括对以下方面的无知:

physical I/O minimization strategies, given that most queries' bottleneck is I/O not CPU perf impact of different kinds of physical storage access (e.g. lots of sequential I/O will be faster than lots of small random I/O, although less so if your physical storage is an SSD!) how to hand-tune a query if the DBMS produces a poor query plan how to diagnose poor database performance, how to "debug" a slow query, and how to read a query plan (or EXPLAIN, depending on your DBMS of choice) locking strategies to optimize throughput and avoid deadlocks in multi-user applications importance of batching and other tricks to handle processing of data sets table and index design to best balance space and performance (e.g. covering indexes, keeping indexes small where possible, reducing data types to minimum size needed, etc.)

没有注释的存储过程或函数…

使用无意义的表别名:

from employee t1,
department t2,
job t3,
...

使得阅读一个大的SQL语句比它需要的要困难得多

我见过太多人死死抓着IN(…)，却完全忘记了EXISTS(存在)。有关一个好例子，请参见Symfony Propel ORM。