Random elements inside JOIN - mysql

Question

I have this code here
INSERT INTO Directory.CatalogTaxonomy (`CatalogId`, `TaxonomyId`, `TaxonomyTypeId`, `IsApprovalRelevant`)
SELECT cat.CatalogId, dep.Id, #department_type, false
FROM Directory.Catalog cat
JOIN (SELECT * FROM (
SELECT * FROM Taxonomy.Department LIMIT 10
) as dep_tmp ORDER BY RAND() LIMIT 3) AS dep
WHERE cat.CatalogId NOT IN (SELECT CatalogId FROM Directory.CatalogTaxonomy WHERE TaxonomyTypeId = #department_type)
AND cat.UrlStatus = #url_status_green
AND (cat.StatusId = #status_published
OR cat.StatusId = #status_review_required);
And the problem is that, it should for each catalog take the first 10 elements from Department and randomly choose 3 of them, then add to CatalogDepartment 3 rows, each containing the catalog id and a taxonomy id. But instead it randomly chooses 3 Department elements and then adds those 3 elements to each catalog.
The current result looks like this:
1 000de9d7-af8b-4bac-bdbd-e6e361e5bc5e
1 001d4060-2924-4c75-b304-d780454f261b
1 001bc4b8-c1bc-498d-9aee-3825a40587d5
2 000de9d7-af8b-4bac-bdbd-e6e361e5bc5e
2 001d4060-2924-4c75-b304-d780454f261b
2 001bc4b8-c1bc-498d-9aee-3825a40587d5
3 000de9d7-af8b-4bac-bdbd-e6e361e5bc5e
3 001d4060-2924-4c75-b304-d780454f261b
3 001bc4b8-c1bc-498d-9aee-3825a40587d5
As you can see, there are only 3 departments chosen and repeated for every catalog

Answer 1

If you think that the query:
SELECT * FROM (
SELECT * FROM Taxonomy.Department LIMIT 10
) as dep_tmp
ORDER BY RAND() LIMIT 3
that you join to Directory.Catalog returns 3 different departments for each catalog then you are wrong.
This query is executed only once and returns 3 random departments which are joined (always the same 3) to Directory.Catalog.
What you can do is after you CROSS JOIN 10 departments to Directory.Catalog, choose randomly 3 of them for each catalog.
Try this:
INSERT INTO Directory.CatalogTaxonomy (`CatalogId`, `TaxonomyId`, `TaxonomyTypeId`, `IsApprovalRelevant`)
WITH cte AS (
SELECT cat.CatalogId, dep.Id AS TaxonomyId, #department_type AS TaxonomyTypeId, false AS IsApprovalRelevant
FROM Directory.Catalog AS cat
CROSS JOIN (SELECT * FROM Taxonomy.Department LIMIT 10) AS dep
WHERE cat.CatalogId NOT IN (SELECT CatalogId FROM Directory.CatalogTaxonomy WHERE TaxonomyTypeId = department_type)
AND cat.UrlStatus = #url_status_green
AND (cat.StatusId = #status_published OR cat.StatusId = #status_review_required);
)
SELECT t.CatalogId, t.TaxonomyId, t.TaxonomyTypeId, t.IsApprovalRelevant
FROM (
SELECT *, ROW_NUMBER() OVER (PARTITION BY CatalogId ORDER BY RAND()) rn
FROM cte
) t
WHERE t.rn <= 3
Note that this:
SELECT * FROM Taxonomy.Department LIMIT 10
does not guarantee that you get the first 10 elements from Department because a table is not ordered.