Best way to structure SQL queries with many inner joins? - mysql

Question

I have an SQL query that needs to perform multiple inner joins, as follows:
SELECT DISTINCT adv.Email, adv.Credit, c.credit_id AS creditId, c.creditName AS creditName, a.Ad_id AS adId, a.adName
FROM placementlist pl
INNER JOIN
(SELECT Ad_id, List_id FROM placements) AS p
ON pl.List_id = p.List_id
INNER JOIN
(SELECT Ad_id, Name AS adName, credit_id FROM ad) AS a
ON ...
(few more inner joins)
My question is the following: How can I optimize this query? I was under the impression that, even though the way I currently query the database creates small temporary tables (inner SELECT statements), it would still be advantageous to performing an inner join on the unaltered tables as they could have about 10,000 - 100,000 entries (not millions). However, I was told that this is not the best way to go about it but did not have the opportunity to ask what the recommended approach would be.
What would be the best approach here?

Answer 1

To use derived tables such as
INNER JOIN (SELECT Ad_id, List_id FROM placements) AS p
is not recommendable. Let the dbms find out by itself what values it needs from
INNER JOIN placements AS p
instead of telling it (again) by kinda forcing it to create a view on the table with the two values only. (And using FROM tablename is even much more readable.)
With SQL you mainly say what you want to see, not how this is going to be achieved. (Well, of course this is just a rule of thumb.) So if no other columns except Ad_id and List_id are used from table placements, the dbms will find its best way to handle this. Don't try to make it use your way.
The same is true of the IN clause, by the way, where you often see WHERE col IN (SELECT DISTINCT colx FROM ...) instead of simply WHERE col IN (SELECT colx FROM ...). This does exactly the same, but with DISTINCT you tell the dbms "make your subquery's rows distinct before looking for col". But why would you want to force it to do so? Why not have it use just the method the dbms finds most appropriate?
Back to derived tables: Use them when they really do something, especially aggregations, or when they make your query more readable.
Moreover,
SELECT DISTINCT adv.Email, adv.Credit, ...
doesn't look to good either. Yes, sometimes you need SELECT DISTINCT, but usually you wouldn't. Most often it is just a sign that you haven't thought your query through.
An example: you want to select clients that bought product X. In SQL you would say: where a purchase of X EXISTS for the client. Or: where the client is IN the set of the X purchasers.
select * from clients c where exists
(select * from purchases p where p.clientid = c.clientid and product = 'X');
Or
select * from clients where clientid in
(select clientid from purchases where product = 'X');
You don't say: Give me all combinations of clients and X purchases and then boil that down so I just get each client once.
select distinct c.*
from clients c
join purchases p on p.clientid = c.clientid and product = 'X';
Yes, it is very easy to just join all tables needed and then just list the columns to select and then just put DISTINCT in front. But it makes the query kind of blurry, because you don't write the query as you would word the task. And it can make things difficult when it comes to aggregations. The following query is wrong, because you multiply money earned with the number of money-spent records and vice versa.
select
sum(money_spent.value),
sum(money_earned.value)
from user
join money_spent on money_spent.userid = user.userid
join money_earned on money_earned.userid = user.userid;
And the following may look correct, but is still incorrect (it only works when the values happen to be unique):
select
sum(distinct money_spent.value),
sum(distinct money_earned.value)
from user
join money_spent on money_spent.userid = user.userid
join money_earned on money_earned.userid = user.userid;
Again: You would not say: "I want to combine each purchase with each earning and then ...". You would say: "I want the sum of money spent and the sum of money earned per user". So you are not dealing with single purchases or earnings, but with their sums. As in
select
sum(select value from money_spent where money_spent.userid = user.userid),
sum(select value from money_earned where money_earned.userid = user.userid)
from user;
Or:
select
spent.total,
earned.total
from user
join (select userid, sum(value) as total from money_spent group by userid) spent
on spent.userid = user.userid
join (select userid, sum(value) as total from money_earned group by userid) earned
on earned.userid = user.userid;
So you see, this is where derived tables come into play.