自定义合计函数

Example: Creating and Using a User-Defined Aggregate

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This example illustrates creating a simple user-defined aggregate function SecondMax() that returns the second-largest value in a set of numbers.

Creating SecondMax()

1. Implement the type SecondMaxImpl to contain the ODCIAggregate routines.

   create type SecondMaxImpl as object
   (
     max NUMBER, -- highest value seen so far 
     secmax NUMBER, -- second highest value seen so far
     static function ODCIAggregateInitialize(sctx IN OUT SecondMaxImpl) 
       return number,
     member function ODCIAggregateIterate(self IN OUT SecondMaxImpl, 
       value IN number) return number,
     member function ODCIAggregateTerminate(self IN SecondMaxImpl, 
       returnValue OUT number, flags IN number) return number,
     member function ODCIAggregateMerge(self IN OUT SecondMaxImpl, 
       ctx2 IN SecondMaxImpl) return number
   );
   /
   
   

2. Implement the type body for SecondMaxImpl.

   create or replace type body SecondMaxImpl is 
   static function ODCIAggregateInitialize(sctx IN OUT SecondMaxImpl) 
   return number is 
   begin
     sctx := SecondMaxImpl(0, 0);
     return ODCIConst.Success;
   end;
   
   member function ODCIAggregateIterate(self IN OUT SecondMaxImpl, value IN number) 
   return number is
   begin
     if value > self.max then
       self.secmax := self.max;
       self.max := value;
     elsif value > self.secmax then
       self.secmax := value;
     end if;
     return ODCIConst.Success;
   end;
   
   member function ODCIAggregateTerminate(self IN SecondMaxImpl, returnValue OUT 
   number, flags IN number) return number is
   begin
     returnValue := self.secmax;
     return ODCIConst.Success;
   end;
   
   member function ODCIAggregateMerge(self IN OUT SecondMaxImpl, ctx2 IN 
   SecondMaxImpl) return number is
   begin
     if ctx2.max > self.max then
       if ctx2.secmax > self.secmax then 
         self.secmax := ctx2.secmax;
       else
         self.secmax := self.max;
       end if;
       self.max := ctx2.max;
     elsif ctx2.max > self.secmax then
       self.secmax := ctx2.max;
     end if;
     return ODCIConst.Success;
   end;
   end;
   /
   
   

3. Create the user-defined aggregate.

   CREATE FUNCTION SecondMax (input NUMBER) RETURN NUMBER 
   PARALLEL_ENABLE AGGREGATE USING SecondMaxImpl;
   
   

Using SecondMax()

SELECT SecondMax(salary), department_id
  FROM employees
  GROUP BY department_id
  HAVING SecondMax(salary) > 9000;

转自：http://download.oracle.com/docs/cd/B10501_01/appdev.920/a96595/dci11agg.htm