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Proper rewriting of aggregation functions over explicit joins.
Aggregations over the left or right side of an explicit join were not properly translated in ConvertToComprehensions.convertSimpleGrouping. We still have a design problem here: In SQL, a count over a single column disregards NULL values whereas a count(*) counts all rows of a view. This distinction should not exist in collection semantics (and it cannot be implemented in ConvertToComprehensions, long after TableRefExpansions have been eliminated, since there is no way of telling a count of a single column from a count of a view of which we only use a single column later). For now, the fix is to count the first column of the projection. This is in line with the behaviour in 1.0.0, allows the correct handling of single-column projections and produces the right result in most cases of counting joined tables. We should improve this further when we add proper support for nested collections. Fixes issue #135. Test in AggregateTest.testGroupBy.
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