5.%20Aggregation%20and%20Negation

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5. Aggregation and Negation

• 5.1 Set grouping
• R0(y1,,ym) R(x1,xn)
• where each yi is either xj or ltxjgt.
• lt gt is similar to GROUP BY in SQL.
• Example
• Reach(x, ltygt) Can_Send_Message(x, y).

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• 5.2 Average, Count, Sum
• These require the aggregated attribute(s) to be
• constants in each constraint tuple.
• Example
• Reach(x, countltygt) Can_Send_Message(x, y).

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• 5.3 Area and Volume
• Example
• Town_Area(t, arealtx,ygt) Town(t, x, y).
• 5.4 Minimum and Maximum
• Aggregation argument a linear function
• Example
• Profit(maxlt30x125x28x315x4gt) Crops(x1, x2,
  x3, x4).

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• 5.5 Negation
• R0(x1,,xk) not R(x1,,xk).
• Example
• Not_Covered(x, y) not Covered(x, y).

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• 5.6 Stratified Datalog Queries
• A query is evaluated and its output (semantics)
• will be an input to another query.
• Example
• Faster(x, y, minlttgt) Travel(x, y, t).
• Travel is the output of a query Qtravel. Now it
  is also
• the input to the above query Qfastest.
• The entire query is written Qfastest(Qtravel()).