How to select own entries and entries of friends in one query - mysql

Question

I am often confronted with such db queries:
Get all entries (e.g. comments) of userX and also all entries of the friends of userX
Which is the best way to do this in SQL (MySQL), assuming userX is not friend of himself.
1. Make two queries and merge them later with PHP
a = SELECT *
FROM comments
WHERE user = X
b = SELECT c.*
FROM comments c
INNER JOIN relation r ON r.user2 = c.user
WHERE r.user1 = X
merge(a, b)
That is what I have usually done. It is rather performant, but I cannot use things as ORDER BY or LIMIT
2. Subqueries with IN and UNION
SELECT c.*
FROM comments
WHERE user IN (
SELECT "X"
UNION
SELECT user2 FROM relation WHERE user1 = X
)
This seems to be very slow, and therefore a bad idea, isn't it?
3. Other solutions? Conditional Joins or something...

Answer 1

One way to do this is to start the query from the user table. Join it with the relation table. Then you can join comments on this with a conditional 'ON' statement. This way MySQL can use the indexes.
select c.* from users a
left outer join relation friend on a.id = friend.user1_id
join comments c on (c.user_id = a.id or c.user_id = friend.user2_id)
where a.id = 1
group by c.id;
Here's a working example: http://sqlfiddle.com/#!2/da298/1

Answer 2

Why not:
SELECT c.*
FROM comments
WHERE user = X OR user IN (
SELECT user2 FROM relation WHERE user1 = X
)
If that's slow, you should look at the execution plan. You might be missing an index.

Answer 3

There is nothing wrong with your #2. You most likely don't have the needed indexes.
As an academic case study, here are various forms that should return the same result set. You should try each one with the EXPLAIN and see which performs best for you (making sure the correct indexes are there) (I'm partial to the third because I have found it perform just as well if not better AND it allows for multiple fields to be related. You can search SO or Google for articles about MySQL performance of IN vs EXISTS)
UNION the results - this is an improved version of your #1; the data is still UNIONed in MySQL, not PHP
SELECT *
FROM comments
WHERE user = X
UNION
SELECT c.*
FROM comments c
INNER JOIN relation r
ON r.user2 = c.user
WHERE r.user1 = X
Your option #2 - IN
SELECT c.*
FROM comments
WHERE user IN (
SELECT "X"
UNION
SELECT user2 FROM relation WHERE user1 = X)
EXISTS
Generally, I prefer this to the IN since it allows for multiple fields to be related
SELECT c.*
FROM comments AS c
WHERE c.user = X
OR EXISTS(SELECT *
FROM relation AS r
WHERE r.user1 = X
AND r.user2 = c.user)