There is no real delete in Redash, instead, when you feel that the query is no longer needed, and it just occupies space in the queries list, you can archive it.

From the users perspective, archiving is almost the same as deleting, since the visualizations of this query will be removed from dashboards.

The only difference is that archived queries will still be reachable through direct links (the URL of the query).

To archive a query, choose Archive from the query menu:

After you choose Archive, Redash will ask you if you're sure that you want to archive the query.

Click on Archive if you're sure, or on Cancel otherwise:

Now, your query won't appear in the queries list, and all the dashboard visualizations of it will be deleted.

If you have a direct URL of the query, however, you will be able to see it (with Archived next to its name):