SQL DB Query - Primary keys

SQL DB Query - Primary keys

 
I created a db query monitor for the following:
 
select
CASE WHEN max(ExecutionFinish) < DATEADD(MINUTE, -15, CURRENT_TIMESTAMP) THEN 1 ELSE 0 END from sysDataCache
 
Warning: Unable to find the primay key for sysDataCache. To show the query output alteast one primary key has to be selected.To select a primary key click the "Primary Keys" link in the top of the sysDataCache table displayed.

 
Why do I get a warning even though I've selected a Primary Key?

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