Timo Walther commented on FLINK-4604:

The problem is that the rule translates not only into {{SUM}} and {{COUNT}} but 
also into {{SqlKind.SUM0}}/{{SqlSumEmptyIsZeroAggFunction}} which is not yet 
supported. This means we need to adapt the {{AggregateUtil}} to support this 
aggregation first. If I use the normal sum aggregator for it, I get the "Type 
NULL is not supported" exception. This is bug actually a null within a {{CASE}} 
expression should have a type.

> Add support for standard deviation/variance
> -------------------------------------------
>                 Key: FLINK-4604
>                 URL: https://issues.apache.org/jira/browse/FLINK-4604
>             Project: Flink
>          Issue Type: New Feature
>          Components: Table API & SQL
>            Reporter: Timo Walther
>            Assignee: Anton Mushin
>         Attachments: 1.jpg
> Calcite's {{AggregateReduceFunctionsRule}} can convert SQL {{AVG, STDDEV_POP, 
> STDDEV_SAMP, VAR_POP, VAR_SAMP}} to sum/count functions. We should add, test 
> and document this rule. 
> If we also want to add this aggregates to Table API is up for discussion.

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