Mechanism behind GROUP BY in SQL considering the Order of Execution - mysql

Question

Considering the "SQL order of execution", how is it possible for the GROUP BY statement to work on a column that is to be made by the CASE WHEN operation in the SELECT statement?
example query:
SELECT
CASE
WHEN COM_SALES_PRC > 1000000 THEN "good job"
END AS Performance,
COUNT(CUST_SEQ_NO) AS NumberofEmployees
FROM PREP_MONTHLY_STAT
GROUP BY Performance
While the above query is ran without an error, the one below gives me an error. Makes sense since WHERE statement would not know the alias to be given in the SELECT statement.
SELECT
CASE
WHEN COM_SALES_PRC > 1000000 THEN "good job"
END AS Performance,
COUNT(CUST_SEQ_NO) AS NumberofEmployees
FROM PREP_MONTHLY_STAT
WHERE Performance IS NOT NULL
SQL order of execution:
https://www.sisense.com/blog/sql-query-order-of-operations/
FROM
WHERE
GROUP BY
HAVE
SELECT
ORDER BY
LIMIT
I'm pretty new to writing queries, and it would be very grateful if the answers are as detailed as possible. Thank you.

Answer 1

First, be aware that the existence of COUNT(...) implies a GROUP BY. If you don't have one explicitly, then then entire tables is one "group". COUNT, SUM, etc are "aggregates".
WHERE cannot refer to expressions in the SELECT. HAVING and ORDER BY can refer to aggregates only after GROUP BY has aggregated them.
WHERE Performance IS NOT NULL could be changed to HAVING Performance IS NOT NULL
(I don't like having SELECT as 5.)
Some issues with your use of COUNT, plus more examples:
COUNT(x) checks x for being NOT NULL. Is that necessary in your case? If not, then simply say COUNT(*).
You are probably getting the total number of employees. If you just wanted the "good job" employees, then
SELECT
COUNT(*) AS GoodJobEmployees
FROM PREP_MONTHLY_STAT
WHERE COM_SALES_PRC > 1000000
If you wanted both the total and the "good job" counts:
SELECT
SUM(COM_SALES_PRC > 1000000) AS GoodJobEmployees,
COUNT(*) AS TotalEmployees
FROM PREP_MONTHLY_STAT
If your real goal is more complex,let's see it.