Sql query optimization for calculating ratio

Question

I have 2 tables (it is only part of table columns that are needed for this query):

Items: Id int, OwnerId int
ItemsRelations: RelationId int, ItemId Int (FK Items(Id))

For each OwnerId i need to calculate result of ratio function: Count of user elements that have links to ItemsRelationstable / count of all user elements.

I wrote this query, but it is slow and not optimal. Is there easier and faster method to do that?

SELECT OwnerId , (100 * SUM(HasRelation))/COUNT(Id) AS Ratio
FROM (
  SELECT 
    oi.OwnerId,
    oi.Id,
    (CASE WHEN SUM(ir.Id) > 0 THEN 1 ELSE 0 END) HasRelation
  FROM Items AS oi
  LEFT JOIN ItemsRelations AS ir ON ir.ItemId  = oi.Id
  GROUP BY oi.Id, oi.OwnerId) tempTab 
GROUP BY OwnerId 

Answer 1

SELECT oi.OwnerId, Ratio = 1.0 * Count(ir.ItemId) / Count(*)
FROM Items AS oi
LEFT JOIN ItemsRelations AS ir ON ir.ItemId  = oi.Id
GROUP BY oi.OwnerId

Notes:

• 1.0 is added to convert the expression into a float result
• Count(ir.ItemId) skips NULLS, so it counts only those that have a relationship

If Items to ItemsRelations is one to many, then you may need to DISTINCT it

SELECT oi.OwnerId, Ratio = 1.0 * Count(DISTINCT ir.ItemId) / Count(DISTINCT oi.Id)
FROM Items AS oi
LEFT JOIN ItemsRelations AS ir ON ir.ItemId  = oi.Id
GROUP BY oi.OwnerId

Answer 2

I think this should do the trick:

select i.OwnerId,
       case when count(i.Id) > 0 then count(r.Id)/count(i.Id) else 0 end as Ratio
from Items i
left join ItemsRelations r on r.ItemId = i.Id
group by i.Id, i.OwnerId