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Relational Databases The Relational Model Jennifer Widom The Relational Model Used by all major commercial database systems Very simple model Query with high-level languages: simple yet expressive Efficient implementations Jennifer Widom The Relational Model Schema = structural description of relations in database Instance = actual contents at given point in time Jennifer Widom The Relational Model Database set of named relationsof(or tables) in database Schema = structural description relations Each relation has acontents set of named attributes columns) Instance = actual at given point in(or time Each tuple (or row) has a value for each attribute Each attribute has a type (or domain) Jennifer Widom The Relational Model Database set of named relations (or tables)inindatabase Schema– =structural structural description relations database Schema description ofofrelations Each relation has acontents set of named attributes (or columns) Instance actual contents given pointinintime time Instance –=actual atatgiven point Each tuple (or row) has a value for each attribute Each attribute has a type (or domain) Jennifer Widom The Relational Model Database set value of named relations (or Schema structural description relations database Schema NULL – special – =structural description for “unknown” ofofrelations ortables) “undefined” inindatabase Each relation has acontents set of named attributes (or columns) Instance actual contents given pointinintime time Instance –=actual atatgiven point Each tuple (or row) has a value for each attribute Each attribute has a type (or domain) Jennifer Widom The Relational Model Database set value of named relations (or tables) Schema structural description relations database Schema NULL Key – –attribute special – =structural whose description for value “unknown” is unique ofofrelations orin “undefined” eachinin tuple database Each has acontents set of named attributes (or columns) Instance =actual actual contents given point time Instance Or setrelation of –attributes whose combined atatgiven point values inintime are unique Each tuple (or row) has a value for each attribute Each attribute has a type (or domain) Jennifer Widom The Relational Model Creating relations (tables) in SQL Create Table Student(ID, name, GPA, photo) Create Table College (name string, state char(2), enrollment integer) Jennifer Widom The Relational Model Used by all major commercial database systems Very simple model Query with high-level languages: simple yet expressive Efficient implementations Jennifer Widom