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Add booleans support to the CASE statement #1156

Description

@alamb

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Sometimes it is useful to use a CASE statement to evaluate predicates within a project operation

Describe the solution you'd like

I would like to be able to create a CASE statement that has a boolean result

For example the following query should return false but instead returns a runtime error:

> explain select case when 'cpu' != 'cpu' then true else false end;
Execution("CASE does not support 'Boolean'")

A workaround is to use a different type like string:

> explain select case when 'cpu' != 'cpu' then 'yes' else 'no' end;
+---------------+-----------------------------------------+
| plan_type     | plan                                    |
+---------------+-----------------------------------------+
| logical_plan  | Projection: Utf8("no")                  |
|               |   EmptyRelation                         |
| physical_plan | ProjectionExec: expr=[no as Utf8("no")] |
|               |   EmptyExec: produce_one_row=true       |
|               |                                         |
+---------------+-----------------------------------------+
2 rows in set. Query took 0.003 seconds.

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