Pick random observation for each by group in SAS

I have two datasets: form and pool having similar table structure.

a) Dataset form

b) Dataset pool

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The above two datsets have three columns where each Key and Level combination in form dataset have 4 rows.

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Also Sub-Level data in form and pool data is mutually exclusive at Key and Level level.

• Sub-Level values has a particular Level assigned to them i.e. Level and Sub-Level follows a hierarchy.

I want to populate the null values under Sub-Level column in form dataset with Sub-Level values in pool dataset where a Sub-Level in pool dataset shoud belong to same Key and Level of form dataset.

How can be this done in SAS ?