I do not think you will be able to. Dataflow SQL uses a variant of ZetaSQL and even with that only supports a subset. Below is the supported functions:
ZetaSQL itself does have an
ARRAY_AGG function, however it does not seem to be supported yet in Dataflow SQL.
Separately what is the use case for the Dataflow engine here, typically you would use it to gain access to querying a pubsub subscription directly for streaming analysis.
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