group by in case of nested cases with conditions on different tables - sql

Question

My question is a bit similar to this question but with a caveat. In my case the conditions are dependent on different tables, not one table. The part which is giving me trouble is the GROUP BY part. Here is the query:
SELECT
CASE
WHEN T1.ImportantColumn = 'Y'
THEN 'Good'
ELSE
CASE
WHEN T2.ImportantColumn = 1
THEN 'Very Good'
ELSE
CASE
WHEN T3.ImportantColumn IS NULL
THEN 'Bad'
ELSE T3.ImportantColumn
END
END
END AS WorkStatus,
SUM(case when T2.sex = 'M' THEN 1 ELSE 0 END) male ,
SUM(case when T2.sex = 'F' THEN 1 ELSE 0 END) female ,
COUNT(WorkStatus) AS [CountWorkStatus]
FROM
Condition1Table T1
RIGHT JOIN Condition2Table T2 ON T1.city = T2.Code_id AND T1.field_name = 'cities'
INNER JOIN Condition3Table T3 ON T2.student_id = T3.student_id
GROUP BY T3.ImportantColumn, T2.ImportantColumn, T1.ImportantColumn -- <-- wrote this but I know it's wrong
It is sort of IF ELSE scenario. If Condition1Table.ImportantColumn is 'Y' then 'Good', else if Condition2Table.ImportantColumn is 1 then 'Very Good', else if Condition3Table.ImportantColumn is NULL then 'bad', Else the value in Condition3Table.ImportantColumn. The hard part is the grouping of data in a desired format which is below:
WorkStatus | male | female | CountWorkStatus
---------- ----- ------ ---------------
Good | 3 | 7 | 10
Very Good | 11 | 2 | 13
Bad | 5 | 0 | 5
Val1 | 1 | 9 | 10
Val2 | 41 | 23 | 64

Answer 1

You seem to be asking "how do I group by a huge CASE statement without repeating the whole CASE statement"?
If so, just use a sub-query.
Then the result of the CASE statement has a column name that you can refer to.
There is near zero performance penalty here, sub-queries are expanded out macro-like. SQL is a declarative language, it's just a syntax for expressing a problem to be solved. When that's compiled down there's a program to run. So, while thinking about the SQL, you just need the syntax to express your problem.
SELECT
WorkStatus,
SUM(case when sex = 'M' THEN 1 ELSE 0 END) male ,
SUM(case when sex = 'F' THEN 1 ELSE 0 END) female ,
COUNT(WorkStatus) AS [CountWorkStatus]
FROM
(
SELECT
CASE
WHEN T1.ImportantColumn = 'Y'
THEN 'Good'
ELSE
CASE
WHEN T2.ImportantColumn = 1
THEN 'Very Good'
ELSE
CASE
WHEN T3.ImportantColumn IS NULL
THEN 'Bad'
ELSE T3.ImportantColumn
END
END
END AS WorkStatus,
T2.sex
FROM
Condition1Table T1
RIGHT JOIN Condition2Table T2 ON T1.city = T2.Code_id AND T1.field_name = 'cities'
INNER JOIN Condition3Table T3 ON T2.student_id = T3.student_id
)
AS StatusBySex
GROUP BY
WorkStatus