How can I aggregate columns of a dataset by a function (mean, min, etc.)?
How do I get the frequency counts of a column in a table?
Select your column in the "group" menu, then pick the "count" aggregation method for that column in the "attributes" menu of the table. See the Table page for more details.
I want to change the SQL query for a dataset in a canvas. How can I do that?
The SQL dataset operator allows you to do this.