MYSQL sum() for distinct rows

Question

I\'m looking for help using sum() in my SQL query:

SELECT links.id, 
       count(DISTINCT stats.id) as clicks, 
       count(DISTINCT conversions.id) as con         


        

Answer 1

I may be wrong but from what I understand

• conversions.id is the primary key of your table conversions
• stats.id is the primary key of your table stats

Thus for each conversions.id you have at most one links.id impacted.

You request is a bit like doing the cartesian product of 2 sets :

[clicks]
SELECT *
FROM links 
LEFT OUTER JOIN stats ON links.id = stats.parent_id 

[conversions]
SELECT *
FROM links 
LEFT OUTER JOIN conversions ON links.id = conversions.link_id 

and for each link, you get sizeof([clicks]) x sizeof([conversions]) lines

As you noted the number of unique conversions in your request can be obtained via a

count(distinct conversions.id) = sizeof([conversions])

this distinct manages to remove all the [clicks] lines in the cartesian product

but clearly

sum(conversions.value) = sum([conversions].value) * sizeof([clicks])

In your case, since

count(*) = sizeof([clicks]) x sizeof([conversions])
count(*) = sizeof([clicks]) x count(distinct conversions.id)

you have

sizeof([clicks]) = count(*)/count(distinct conversions.id)

so I would test your request with

SELECT links.id, 
   count(DISTINCT stats.id) as clicks, 
   count(DISTINCT conversions.id) as conversions, 
   sum(conversions.value)*count(DISTINCT conversions.id)/count(*) as conversion_value 
FROM links 
LEFT OUTER JOIN stats ON links.id = stats.parent_id 
LEFT OUTER JOIN conversions ON links.id = conversions.link_id 
GROUP BY links.id 
ORDER BY links.created desc;

Keep me posted ! Jerome

Answer 2

For an explanation of why you were seeing incorrect numbers, read this.

I think that Jerome has a handle on what is causing your error. Bryson's query would work, though having that subquery in the SELECT could be inefficient.