anchor.diff:  [NOTE]
   `_g` is an implicit variable for `agg` aggregate function,
   it represents the grouped `Queryable` object and its record(e.g. `r`) could reference the data source by alias(e.g. `n`)
  -
  +////
   
   Also, we could apply the aggregate functions for the whole GINQ result, i.e. no `groupby` clause is needed:
   [source, groovy]
  
positive.code: |java.math.BigDecimal
  |the statistical variance for the population for all non-null values
  
  |agg(_expression_)
  |any
  |any
  |customizes the aggregation logic in _expression_ and returns single value
  
  |===
  
  
negative.code: include::../test/org/apache/groovy/ginq/GinqTest.groovy[tags=ginq_grouping_12,indent=0]
  ----
  
  ////
  [source, sql]
  ----
  include::../test/org/apache/groovy/ginq/GinqTest.groovy[tags=ginq_grouping_03,indent=0]
  