人们在使用SQL时往往会陷入一个误区，即太关注于所得的结果是否正确，而忽略了不同的实现方法之间可能存在的 性能差异，这种性能差异在大型的或是复杂的数据库环境中（如联机事务处理OLTP或决策支持系统DSS）中表现得尤为明 显。笔者在工作实践中发现，不良的SQL往往来自于不恰当的索引设计、不充份的连接条件和不可优化的where子句。在对 它们进行适当的优化后，其运行速度有了明显地提高！下面我将从这三个方面分别进行总结：-people in the use of SQL often fall into the trap, that is too focused on the findings is correct, and ignore a different method may exist between the performance differences, these differences in large or complex database environment (such as online transaction processing or decision support OLTP system DSS) has become even more obvious. In practice I found that the poor tend to SQL from inappropriate design of the index, not fully connected and non-optimal conditions where the phrase. On their proper optimization, and their operating speed has been improved! Now I will be three aspects of this summary :