Last Tested: Jan 21, 2019
Create filters by adding those filters in the source queries, either when creating and merging the queries, or by editing the queries from the Merged Results page. Note that you can’t add filter directly to the merged results. More information can be found here: Looker Document
For filters post-merge, we can use a yesno if() function in table calculation and hide 'no's from the visualization.
You can also apply a dashboard-level filter to any of the source queries, as noted in step 6 of this doc.
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