How to use complicated nested case statements?

heyday2004
heyday2004 used Ask the Experts™
on
I have a SQL statement to calculate some stats data for analytical report purpose, it's working good:

select
SUM(CASE WHEN (createDate = CURRENT_DATE AND exitDate IS NULL) THEN 1 ELSE 0 END)
...
from sample table
group by ...

Now I want to add more complicated logic to the script (adding nested "AND" and "OR" logic), something like:

select
SUM(CASE WHEN (createDate = CURRENT_DATE) AND (exitDate IS NULL OR (exitDate > CURRENT_DATE)) THEN 1 ELSE 0 END) AS survey_Tasks_Started_And_Not_Completed,
...
from sample table
group by...

The purpose is, when createDate = CURRENT_DATE, I want to include records of both (exitDate field is null) or (exitDate is later than today)

But seems it's not working if I add too many parenthesis and nested statements... Any solution for this kind of issue when adding complicated nested case statements in MySQL?

Or is there any recommendation for better solution for such common scenarios since too many nested level cause issue too?

Thanks!
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Dale BurrellDirector
Commented:
No reason that I know of for that not to work, of you post the actual query, and the results that suggest its not working and maybe someone can help further.
Aaron TomoskyDirector of Solutions Consulting
Commented:
When my stuff gets complex I use CTE to clean it up.

With stuff as (
Select
(case when blah and blah then 1) as startedandnotcompleted from sampletable
)
Select count(*) from stuff where startedandnotcompleted = 1

Author

Commented:
Thanks, any further suggestion on the best practice to deal with such complicated nested scenario? I think it's very error-prone.  Thanks.
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Dale BurrellDirector
Commented:
There's nothing wrong with using complex/nested case statements, its a perfectly good solution to the right problem. All SQL is error prone - its very easy to get a query that works without giving you the data you want. Please post you query and the incorrect result as its not really possible to help you further in a general sense.
Aaron TomoskyDirector of Solutions Consulting
Commented:
Agreed, nothing wrong with complex nested case, I think the issue becomes when you add the group by and such. The trick to complex SQL is to do it one step at a time. With sub querries or CTE, it lets you do one thing at a time, see the results, an build the next step off it.
Information Technology Specialist
Commented:
No need for nesting conditions, just stack them -
SUM(CASE WHEN createDate = CURRENT_DATE AND exitDate IS NULL THEN 1
                 WHEN createDate = CURRENT_DATE AND exitDate > CURRENT_DATE THEN 1
                 ELSE 0
         END) AS survey_Tasks_Started_And_Not_Completed,
...
Note - when using case like this, it is always best to list the condition most likely to occur first, second most second, etc.

Author

Commented:
Thanks.

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