-
relational model
- Massive,Persistent,Safe,Multi-user,Convenient,Efficient,Reliable
- --------------------------------------------------------------------
- Database = set of named relations(or tables)
- Each relation has a set of named attributes(or columns)
- Each tuple(or row) has a value for each attribute
- Each attribute has a type(or domain)
- Schema--structural description of relations in database
- Instance--actual contents at given point in time
- NULL--special value for "unknown" or "indefined"
- key--attribute whose value is unique in each tuple or set of attributes whose combined values are unique.
- --------------------------------------------------------------------
- relation指的是表中各属性之间的关系,而不是表与表之间的关系
- xml:tag,attribute,text;DOM,SAX;CSS,XSL;DTD,XSD;
- xml用来存储结构化数据
- --------------------------------------------------------------------
- JSON:存储半结构化数据,base values+Objects{}+Arrays[]
- ----------------------------------------------------------------------
- Relational Algebra,http://www.youtube.com/user/dglearn?feature=watch
- cross join,natural join,theta join,union,difference,intersection,rename,alternate
-
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原文地址:https://www.cnblogs.com/dmdj/p/3524697.html
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