Calculations using aliases (from a subquery to the same table) mySQL - mysql

Question

I have a database that stores player kills in CS:GO, I am trying to write a query that can show each player's KD.
I've written a query that will show each player's kills and deaths using aliases.
SELECT
`Name`,
`SteamID` as PlayerID,
count(`EventType`) as kills,
(SELECT count(`EventType`)
FROM `logdata`
WHERE (`EventVariable` = PlayerID AND `EventType` = 'killed')
GROUP BY `EventVariable`
ORDER BY `count(``EventType``)` DESC) as deaths
FROM `logdata`
WHERE `EventType` = 'killed'
GROUP BY `EventType`, `Name`
ORDER BY kills DESC
(results limited to just bots, I didn't want to openly advertise my friends SteamIDs)
To work out KD I just need to divide kills / deaths but you can't do that with aliases, I read that I should be able to wrap the alias e.g. (SELECT kills) / (SELECT deaths) as KD but that doesn't work.
The table looks like this: (Limited to bots again)
I am currently working out KD in PHP using the result of my query but that isn't a great way of doing it. (I am unable to query who has the highest KD for example)
So, my question is, how would I go about calculating the KD if I am unable to make calculations using alias?

Answer 1

I might just write your query using two completely separate subqueries which compute the kills and deaths counts:
SELECT
n.Name,
COALESCE(t1.kill_cnt, 0) AS kills,
COALESCE(t2.death_cnt, 0) AS deaths,
CASE WHEN t2.death_cnt > 0
THEN CAST(t1.kill_cnt / t2.death_cnt AS CHAR(50))
ELSE 'NA' END AS ratio
FROM
( SELECT DISTINCT Name FROM logdata ) n
LEFT JOIN
(
SELECT Name, COUNT(*) AS kill_cnt
FROM logdata
WHERE EventType = 'killed'
GROUP BY Name
) t1
ON
n.Name = t1.Name
LEFT JOIN
(
SELECT EventVariable AS Name, COUNT(*) AS death_cnt
FROM logdata
WHERE EventType = 'killed'
GROUP BY Name
) t2
ON
n.Name = t2.Name
Note that the subquery above which I have aliased as n is just intended to generate a complete list of all users in your database. Ideally, there should be a dedicated user table somewhere. If not, and you don't like my approach, then you will have to come up with some other way to obtain a list of all users.

Answer 2

Thanks to Tim for pointing me in the right direction and providing a query. I have made some changes to get the result I want and I wanted to post the final result.
SELECT
n.SteamID,
COALESCE(t1.kill_cnt, 0) AS kills,
COALESCE(t2.death_cnt, 0) AS deaths,
CASE WHEN t2.death_cnt > 0 THEN CAST(t1.kill_cnt / t2.death_cnt AS CHAR(50))
WHEN t1.kill_cnt = 0 THEN '0'
ELSE 'Infinite' END AS ratio
FROM
( SELECT DISTINCT SteamID FROM logdata ) n
LEFT JOIN
(
SELECT SteamID, COUNT(*) AS kill_cnt
FROM logdata
WHERE EventType = 'killed'
GROUP BY SteamID
) t1
ON
n.SteamID = t1.SteamID
LEFT JOIN
(
SELECT EventVariable AS SteamID, COUNT(*) AS death_cnt
FROM logdata
WHERE EventType = 'killed'
GROUP BY EventVariable
) t2
ON
n.SteamID = t2.SteamID
WHERE t1.kill_cnt > 0 or t2.death_cnt > 0
ORDER BY `ratio` DESC
I attempted to get KD 0 to show as such but that is not all that important at the end of the day, NULL is easy to work with.