请问sql中的数据字典是什么，起到什么作用？

就是维护整个数据库运行的内部表。属于数据库系统的表。
oracle官方的解释是

One of the most important parts of an Oracle database is its data dictionary, which is a read-only set of tables that provides information about the database. A data dictionary contains:

The definitions of all schema objects in the database (tables, views, indexes, clusters, synonyms, sequences, procedures, functions, packages, triggers, and so on)

How much space has been allocated for, and is currently used by, the schema objects

Default values for columns

Integrity constraint information

The names of Oracle users

Privileges and roles each user has been granted

Auditing information, such as who has accessed or updated various schema objects

Other general database information

The data dictionary is structured in tables and views, just like other database data. All the data dictionary tables and views for a given database are stored in that database's SYSTEM tablespace.

说到数据字典，就是相当于汉字对于字典而言，只不过数据字典是在你写需求的时候写的，方便以后写数据的时候用到！

数据流图就是用一些不同意义的几何图形表示数据库里面不同的结构，然后就构成的数据流图。
数据字典我们所用到的一般就是将数据库里面的字段用表格形式表现出来的。