Make a calculation with aliases in one (sub)query - mysql

Question

I have a table with data and am trying to make the following calculation in a query: IDO / IDP * 100.
**TBL**
Name IDP IDO
ABC 123 231
DEF 124 NULL
GHI 125 NULL
JKL 126 342
KNM 127 NULL
I managed to count up all the values needed to make the calculation and gave each an alias. This is what I have so far, but the calculation part returns an error telling me the references are not supported.
SELECT
SUM(case when IDP is not null then 1 end) as Checked,
SUM(case when IDO is null then 1 end) as NotChecked,
(SELECT Checked / NotChecked) * 100 AS Result
FROM TBL
I've also tried with a subquery, but it results in the same error.
SELECT
SUM(case when IDP is not null then 1 end) as Checked,
SUM(case when IDO is null then 1 end) as NotChecked,
(
SELECT Checked / NotChecked * 100 AS Result FROM TBL
)
FROM TBL
What would be the right or a better way to do this?

Answer 1

I don't think you will be able to reference aliases created in a SELECT clause in that same SELECT clause.
As you probably know, you could just repeat the logic, like the following (although I assume the purpose of your question is to avoid writing the logic twice).
SELECT
SUM(case when IDP is not null then 1 end) as Checked,
SUM(case when IDO is null then 1 end) as NotChecked,
(SUM(case when IDP is not null then 1 end) / SUM(case when IDO is null then 1 end)) * 100 AS Result
FROM TBL
A subquery version, where you don't repeat the logic, would be the following. Note that the aliases are created in the subquery so the outer query has no trouble referencing them.
SELECT
Checked, NotChecked, (Checked / NotChecked) * 100 as Result
FROM
(
SELECT
SUM(case when IDP is not null then 1 end) as Checked,
SUM(case when IDO is null then 1 end) as NotChecked
FROM TBL
) tt
Since you don't seem to be summing, just counting null/non-null values, I personally find the sum/case statements a bit confusing and might prefer something like this:
SELECT
Checked, NotChecked, (Checked / NotChecked) * 100 as Result
FROM
(
SELECT
count(IDP) as Checked,
count(*)-count(IDO) as NotChecked
FROM TBL
) tt