Merry Christmas right back at ya!
The way I use the Data Explorer plugin is I have a query named “test” that I use for “alpha” queries and when I find one I like rename it to something more descriptive and save it then create another “test”.
To the right of the input area there is a list of Discourse tables. They can be expanded to show field names with their data types.
I have limited experience with different databases, so my SQL isn’t so bad. But I still check the postgreSQL documentation often
IMHO the plugin does a great job of letting you know when something is wrong with the query syntax. And it has safety built in in case you run a resource hungry query. However, similar to poor regex, you need to get things right. As an analogy, if you tell it “I wants watere” it will give you “plural used incorrectly, no such thing as watere” messages, but if you tell it “I want water” when you want bread, it will happily give you water.
I’m in the habit of explicitly SELECTing the fields I want so I don’t know if the “*” wildcard will work, but it may. eg.
WHERE topic_id IS NULL
should let you know how many of these you have.