几类database system
Hierarchical database
data is represented as one or more tree structures. example (single-parent hierarchy), each node has zero or one parent node and zero, one or many children nodes.
network database system
exposes sets of records and sets of links that define relationships between different records.
The Relational Model
example
primary key 用于识别行的独一无二的数据
compound key 由多列组成的独一无二的识别数据
natural key and surrogate key natural key(如姓名等)相对于而言更容易改变, 而surrogate key由于是数据库设计人员所设定，不会轻易改变。
foreign key 一个或更多列用于在另外的表内识别此行, 如Account table内的cust_id
