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Union, Intersection, Difference Operations

  • The union operator allows us to combine two relations
  • The intersection operator allows us to take the intersection of two relations
  • The difference operator allows us to take the difference of two relations
  • For \(r \cup s\), \(r \cap s\) , or \(r-s\) to be valid,
    • r and s must have the same number of attributes
    • the attribute domains must be compatible with each other

Example

$$\begin {align}

\quad &\Pi_{\,course_ID}\,(\sigma_{semester=^"Fall^" \,\land\,year=2017}\,(section)) \ \ \cup \

\quad &\Pi_{\,course_ID}\,(\sigma_{semester=^"Spring^" \,\land\,year=2018}\,(section)) \

\end {align}$$