Use previous value in MySQL to compute the next field? - mysql

Question

How to use previous values in MySQL to compute the next value? I couldn't explain what I mean more clearly in my poor English (anyone can understand what I asked by seeing the title of this post?), so let me explain it in a snippet:
select
year_id,
sum(case when event_cd >= 20 then 1 else 0 end) as h,
sum(case when ab_fl = "T" then 1 else 0 end) as ab,
h/ab as ba
from events
group by year_id
in the above snippet, when you run the query you will get the error, since there are no such columns in events table as h or ab. However, I want to use h and ab which are computed in the previous sum(case when ~) syntax. I remember there are some sort of ways to make it possible, but I don't remember how to do nor as I said, I couldn't find any relevant posts to meet what I'm asking due to my poor English, though since I'm sure this question is already posted here at SO, it's quite helpful even if you just link it with no detailed explanation.
Thanks.
[Update]
Thanks for the answers. I just wanted to use previous values in order to avoid subqueries and hence lots of redundant typing, and make the entire code more readable. I've used either methods (subqueries or write the entire syntax twice just to compute another value) as described by Mosty Mostacho, and if this is not feasible or pretty risky to use in MySQL, I can surely accept those two methods above. Sorry for the confusion.

Answer 1

There are 2 ways in MySQL (BTW, I've simplified the query a bit but it is still legal):
Option #1: Expand the variables
SELECT
year_id,
SUM(event_cd >= 20) h,
SUM(ab_fl = "T") ab,
SUM(event_cd >= 20) / SUM(ab_fl = "T") ba
FROM events
GROUP BY year_id
Option #2: Use a derived table
SELECT year_id, h, ab, h / ab ba FROM (
SELECT
year_id,
SUM(event_cd >= 20) h,
SUM(ab_fl = "T") ab
FROM events
GROUP BY year_id
) s
You might be tempted to think that the second will run faster but it is not the case. The first one is most likely to run faster because it doesn't need a derived table and can solve the issue in just one pass.

Answer 2

try
select * h/ab as ba from
(
select
year_id,
sum(case when event_cd >= 20 then 1 else 0 end) as h,
sum(case when ab_fl = "T" then 1 else 0 end) as ab,
from events
group by year_id)
source

Answer 3

Update
As #MostyMostacho explains in the comment below, this is not an advisable technique, due to the order of evaluation of user variable being undefined.
You can write a query using a MySQL variable using the syntax #varname := 'value' to store your sums in, and then reference that.
SELECT
year_id
, #h := SUM(CASE WHEN event_cd >= 20 THEN 1 ELSE 0 END) AS h
, #ab := SUM(CASE WHEN ab_fl = "T" THEN 1 ELSE 0 END) AS ab
, #h/#ab AS ba
FROM events
GROUP BY year_id