With the sample data you provided, I don't get results from either form, whether the date is hardcoded or pulled from OT_Jacob, primarily because there's only one row in that table's sample data that has a DateKey matching anything in the Alerts table.

Since that one row is not for the StrikeOne app, and is also not 20170517, nothing gets returned from either query with this sample data.

Whatever you're seeing must be due to the specific data you're querying. Without being able to reproduce the behavior with your sample data, I could only guess that the results you're getting from the first query are results where the DateKey is NOT 20170517. 

The second query restricts results to rows having a DateKey of 20170517, while the first does not, so that would be my guess, but it's tough to say for sure without being able to reproduce the behavior.

Cheers!