在SQL的两个列上计数不相同

Question

Let's consider this example :

Employee     Function   Start_dept   End_dept
A               dev          10        13
A               dev          11        12
A               test          9         9
A               dev          13        11

What I want to select is employee, their function and the distinct departments in BOTH "start" and "end" department. It will give this result :

Employee     Function  count_distinct_dept
A                 dev          4
A                 test         1            `

For the dev A, we have only 4 distinct departments (10, 11, 12 and 13) because we shouldn't count duplicate values in the 2 columns (start and end).

How can I do this ? (I'm using mySQL). Is it possible to do this on one request without any JOIN or any UNION ? Or is it obligatory to use one of them ? Since I am using a huge database (with more than 3 billions lines), I am not sure if a join or union request will be optimal...

Answer 1

Use a union all and aggregation:

select Employee, Function, count(distinct dept)
from ((select Employee, Function, Start_dept as dept
       from e
      ) union all
      (select  Employee, Function, End_dept
       from e
      )
     ) e
group by Employee, Function;

If you want performance, I would suggest starting with two indexes on (Employee, Function, Start_Dept) and (Employee, Function, End_Dept) . Then:

select Employee, Function, count(distinct dept)
from ((select distinct Employee, Function, Start_dept as dept
       from e
      ) union all
      (select distinct Employee, Function, End_dept
       from e
      )
     ) e
group by Employee, Function;

The subqueries should be scanning the index rather than the overall table. You will still need to do the final GROUP BY . I am guessing that COUNT(DISTINCT) is a better approach than UNION in the subquery, but you could test that.