■A1,A2, ...,Anareattributes

■R= (A1,A2, ...,An) is arelation schema

Example:instructor= (ID, name, dept_name, salary)

■Formally, given setsD1,D2, ....Dnarelationris a subset ofD1xD2x ... xDn

Thus, a relation is a set ofn-tuples (a1,a2, ...,an) where each ai belongs to D

• ■The current values (relation instance) of a relation are specified by a

  table

• ■An elementtofris atuple, represented by arowin a table

Relations are Unordered