Question

I have the following scenario:

 Table 1:

 articles

 id article_text  category  author_id
 1  "hello world"    4                1
 2 "hi"              5                2
 3 "wasup"           4                3


 Table 2

 authors

 id   name     friends_with
 1    "Joe"      "Bob"
 2    "Sue"      "Joe"
 3    "Fred"     "Bob"

I want to know the total number of authors that are friends with “Bob” for a given category.

So for example, for category 4 how many authors are there that are friends with “Bob”.

The authors table is quite large, in some cases I have a million authors that are friends with “Bob”

So I have tried:

Get list of authors that are friends with bob, and then loop through them and get the count for each of them of that given category and sum all those together in my code.

The issue with this approach is it can generate a million queries, even though they are very fast, it seems there should be a better way.

I was thinking of trying to get a list of authors that are friends with bob and then building an IN clause with that list, but I fear that would blow out the amt of memory allowed in the query set.

Seems like this is a common problem. Any ideas?

thanks

Answer 1

SELECT COUNT(DISTINCT auth.id)
FROM authors auth
INNER JOIN articles art ON auth.id = art.author_id
WHERE friends_with = 'bob' AND art.category = 4

Count(Distinct a.id) is required as articles might hit multiple rows for each author.

But if you have any control over the database I would use a link table for friends_with as your cussrent solution either have to use a comma seperated list of names which will be disastrous for performance and require a completly different query or each author can only have one friend.

Friends

id friend_id

then the query would look like this

SELECT COUNT(DISTINCT auth.id)
FROM authors auth
INNER JOIN articles art ON auth.id = art.author_id
INNER JOIN friends f ON auth.id = f.id 
INNER JOIN authors fauth ON fauth.id = f.friend_id
WHERE fauth.name = 'bob' AND art.category = 4

Its more complex but will allow for many friends, just remeber, this construct calls for 2 rows in friends for each pair, one from joe to bob and one from bob to joe.

You could build it differently but that would make the query even more complex.