1. Massive,Persistent,Safe,Multi-user,Convenient,Efficient,Reliable
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3. Database = set of named relations(or tables)
4. Each relation has a set of named attributes(or columns)
5. Each tuple(or row) has a value for each attribute
6. Each attribute has a type(or domain)
7. Schema--structural description of relations in database
8. Instance--actual contents at given point in time
9. NULL--special value for "unknown" or "indefined"
10. key--attribute whose value is unique in each tuple or set of attributes whose combined values are unique.
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12. 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
13. xml：tag，attribute，text；DOM，SAX；CSS，XSL；DTD，XSD；
14. xml用来存储结构化数据
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16. JSON：存储半结构化数据，base values+Objects{}+Arrays[]
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18. Relational Algebra,http://www.youtube.com/user/dglearn?feature=watch
19. cross join，natural join，theta join，union，difference，intersection，rename，alternate

relational model