MySQL - Grandchild category aggregation

Question

I have following data hierarchy: Report > has many Modules > has many Instances.... each instance might have a different category assigned to it (finite but unknown amount of categories)

I need to produce a report level aggregate of instance categories... something like this:

| report_id | category  |       #     |
|         1 |         1 |           10|
|         1 |         2 |           5 | 
|         2 |         7 |           2 | 
|         2 |         4 |           14| 

I am new to MySQL and I have tried the following:

mysql> SELECT report_id.report_id, module_id.module_id, instance.instance_id, violation.name
-> FROM report_id
-> LEFT JOIN module_id
-> ON report_id.report_id=module_id.report_id
-> LEFT JOIN instance
-> ON module_id.module_id = instance.module_id
-> LEFT JOIN violation
-> ON instance.violation_id = violation.violation_id
-> ORDER BY report_id.report_id
-> ;

To produce:

+-----------+-----------+-------------+----------------+
| report_id | module_id | instance_id | category       |
+-----------+-----------+-------------+----------------+
|         1 |         1 |           1 | 1              |
|         1 |         2 |           5 | 1              |
|         1 |         1 |           2 | 2              |
|         1 |         1 |           3 | 3              |
|         1 |         1 |           4 | 3              |
|         1 |         3 |           6 | 3              |
|         2 |      NULL |        NULL | NULL           |
|         3 |      NULL |        NULL | NULL           |
+-----------+-----------+-------------+----------------+

I have also tried:

mysql> SELECT violation.name as Category, COUNT(instance.instance_id) AS NumberOfViolations FROM instance
-> LEFT JOIN violation
-> ON instance.violation_id=violation.violation_id
-> GROUP BY name;

+----------------+--------------------+
| Category       | NumberOfViolations |
+----------------+--------------------+
| 1              |                  2 |
| 2              |                  1 |
| 3              |                  3 |
+----------------+--------------------+

But this returns total category count for all the grandchildren. (now I need these totals separated by top level Report group)

I am having trouble finding the right syntax to marry these two queries to produce the table I show about. Any clues as to how to proceed would be helpful.

I do realized that I need someway to introduce SUM if (or count if) WHERE report's ID matched module's ID and module's ID matched instance ID but not sure how to incorporate this nested structure.

Answer 1

Thank you Marc B for making that comment about Grouping, that was the nudge that I needed.

This is what the resulting query looks like:

mysql> SELECT report_id.report_id, violation.name as Category, COUNT(instance.instance_id) AS NumberOfViolations FROM instance
-> LEFT JOIN violation
-> ON instance.violation_id=violation.violation_id
-> LEFT JOIN module_id
-> ON instance.module_id = module_id.module_id
-> LEFT JOIN report_id
-> ON module_id.report_id = report_id.report_id
-> GROUP BY report_id, violation.name;

Please note how I am still using module_id to join Instances (which hold violation_ids) to upper level report_ids but I am not using module_id as part of the SELECT statement so the output skips that column to just show my Report level category aggregation. Output below:

+-----------+----------------+--------------------+
| report_id | Category       | NumberOfViolations |
+-----------+----------------+--------------------+
|         1 | 1              |                  2 |
|         1 | 2              |                  1 |
|         1 | 3              |                  3 |
|         2 | 2              |                  10|
+-----------+----------------+--------------------+