Summary: Functions, Multiple Tables, and Sub-queries总结：函数、多个表和子查询

Congratulations! You have completed this lesson. At this point in the course, you know: 
恭喜！您已完成本课程。此时，您已经知道：

• Tools for database management that offer built-in functions for performing operations on data within the database itself.
  数据库管理工具提供内置函数，用于在数据库本身中对数据进行操作。
• That when working with large datasets, you may save time by using built-in functions rather than first retrieving the data into your application and then executing functions on the retrieved data.
  当处理大型数据集时，使用内置函数可能比先从应用程序中检索数据，然后在检索到的数据上执行函数来节省时间。
• You can use sub-queries to form more powerful queries than otherwise.
  您可以使用子查询来形成比其他情况下更强大的查询。
• You can use a sub-select expression to evaluate some built-in aggregate functions like the average function. 
  您可以使用子选择表达式来评估一些内置的聚合函数，例如平均函数。
• Derived tables or table expressions are sub-queries where the outer query uses the results of the sub-query as a data source.
  派生表或表表达式是子查询，外层查询使用子查询的结果作为数据源。