Question

Suppose I’ve got a table:

Role ---- person_id company_id financial_year 

How can I tell the following:

1. Whether each person_id occurs at most once per company_id per financial_year in this table
2. If 1. is false, which person_id’s and company_id’s and financial_year’s co-occur more than once

Edit 1: Edited to add financial_year col

Edit 2: the RDBMS platform here happens to be MySQL, though I don’t expect that this would require much vendor-specific SQL

Answer 1

For the first, it’s generally a good idea to just have a grouping which you can then filter on if you want:

select   r.company_id, r.person_id, r.financial_year, count(r.person_id)  from   Role as r  group by   r.company_id, r.person_id, r.financial_year 

For the second, you can just modify the above like so:

select   r.company_id, r.person_id, r.financial_year, count(r.person_id)  from   Role as r  group by   r.company_id, r.person_id, r.financial_year having   count(r.person_id) > 1