I have yet to see a cross join represented by a Venn diagram, so this strikes me as a red herring on the topic of whether Venn diagrams for the joins that are usually represented with them make sense.
Say tbl_1 has 5 rows, and column "A" is the numbers 1 to 5. Then tbl_2 has the numbers 1 and 2, each repeated 5 times. Now how many rows are in the result of the query below?
select tbl_1.A from tbl_1 left join tbl_2 using(A)
The diagrams that the author of the article comes up with (with the exception of the cross join) are just Venn diagrams rotated 90 degrees.
select tbl_1.A
from tbl_1
left join tbl_2
on tbl_1.A = tbl_2.A
It is in fact a set difference and the sets in question are values for the column being ONed on in each table (as I explained in the top level comment)
My reply was with respect to these being cross joins with filters. That’s just one way of thinking about a JOIN that actually adds notational complexity (and does not actually represent how your database would be doing the join because it would be unnecessarily expensive computationally). Thinking of it in terms of a set difference of the values in thr ON column is arguably the better way of thinking of it if you are interested in how such a join should be implemented. Alternatively you can interpret the venn diagrams as just the filtration if you prefer interpreting as a cross join with filters.
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In anticipation of some possible technical replies, let me be more explicit:
It isn't a set difference, no matter how you view it.
Viewpoint 1: The original lists of objects are not actually sets because the elements are not distinct. From this viewpoint, it isn't a set difference because they aren't even sets to begin with.
Viewpoint 2: There is an implicit "row number" feature of each object so in that sense the objects are distinct and do form sets. From this viewpoint, it still isn't a set difference. If you have one set with 5 elements and another with 10, the set difference cannot have 13 elements.
The "filtered cross-join" model allows students who have learned SELECT and WHERE to think of a JOIN as an extension of those primitives, a combination of two tables on which they can filter.
Venn diagrams might be useful to visualize an outcome, but they will not support stepping through primitives to a solution.
In "filtered cross-join", JOIN can be an extension of SELECT that combines two tables. The combination is a set of rows, each each of which combines all the fields of one row of one table with all the fields of one row of the second. This is easily visualized with two two-column tables of three rows.
They can then use WHERE to find those rows with matches on the key field.
With this model, students can build up JOIN as an abstraction of simpler primitives. When they are struggling with a problem, you can ask them to first step through those primitives to accumulate the solution. Say, query the "raw" join and examine a few rows to see which they want returned. What is true of those and not true of the ones you do not want? ("I want those where these two fields match, and none that do not.") Ok, how do you express that condition in SQL? ("Where . . . this equals that?" "Hmm, try that" "HEY THAT WORKED!")
With that basis, they have a model that can extend to more complexity -- joins across several tables, joins on the same table, joins with conditions other than field equality.
Building up to and using this model, you can have the vast majority of students writing joins with confidence in two days.
My suggestion to the site would be, use an example that has two columns on each table, to provide the key field on which the join will be performed.
If you think of inner joins as a cross join with a filter, then we have all seen cross joins represented by Venn diagrams - and for me at least I lost several hours of work restructuring my understanding of what I was working with!