SQL Case on Where clause to different columns - sql

Question

I've been trying to get a query that, based on a given condition (if isCurrent = 1 or not) should give me just one value/row based on the CurriculumId /which will be a parameter on a stored procedure).
This value should, in case isCurrent = 1 return to me the item with the most current StartDate but if isCurrent = 0 then it should give me the one with the most current EndDate.
The thing is that I only want one item per CurriculumId, ideally the one with isCurrent = 1 and the most current StartDate (ignoring the remaining rows) but, if there are no experiences with isCurrent = 1, then it should return to me the one with the most current EndDate.
My previous query was almost working but I still got the one with the most current StartDate (isCurrent = 1) AND the one with the most current EndDate when I want to retrieve just one or another.
I've come to the query bellow:
SELECT table.IntProfessionalExperienceId,
table.IsCurrent,
table.StartDate,
table.EndDate
FROM table
WHERE table.CurriculumId = 12
AND
CASE table.IsCurrent
WHEN 1
THEN
table.StartDate = (
SELECT max(table.StartDate)
FROM table
WHERE table.IsCurrent = 1
AND table.CurriculumId = 12
GROUP BY table.CurriculumId
)
ELSE
table.EndDate = (
SELECT max(table.EndDate)
FROM table
WHERE table.CurriculumId = 12
GROUP BY table.CurriculumId
)
END
Individually, the queries seem to be working OK and returning the supposed value although when ran as a whole I get the following errors:
Msg 102, Level 15, State 1, Line 8
Incorrect syntax near '='.
Msg 102, Level 15, State 1, Line 14
Incorrect syntax near ')'.
Msg 102, Level 15, State 1, Line 21
Incorrect syntax near ')'.
What in my syntax is wrong? I know from reading the errors what is wrong with the query but I just don't know how to fix it. And is it just the syntax or am I doing the query wrong to start with?

Answer 1

Split this into multiple conditions, like this:
SELECT
table.IntProfessionalExperienceId,
table.IsCurrent,
table.StartDate,
table.EndDate
FROM table
WHERE
table.CurriculumId = 12 AND
(
(
Table.IsCurrent = 1 AND
table.StartDate =
(
SELECT max(table.StartDate)
FROM table
WHERE
table.IsCurrent = 1 AND
table.CurriculumId = 12
GROUP BY table.CurriculumId
)
) OR
(
ISNULL(table.IsCurrent,0) != 1 AND
table.EndDate =
(
SELECT max(table.EndDate)
FROM table
WHERE table.CurriculumId = 12
GROUP BY table.CurriculumId
)
)
)
EDIT: another, arguably simpler approach would be to pre-aggregate the data you want in your WHERE clause so that you only need to call it a single time, rather than evaluate each row separately. Something like the following:
SELECT
table.IntProfessionalExperienceId,
table.IsCurrent,
table.StartDate,
table.EndDate
FROM
table
INNER JOIN
(
SELECT
MAX(table.EndDate) AS MaxEndDate,
MAX(CASE WHEN table.IsCurrent = 1 THEN table.StartDate END) AS MaxCurrentStartDate
FROM table
WHERE CurriculumID = 12
) MaxDates ON
(Table.IsCurrent = 1 AND Table.StartDate = MaxDates.MaxCurrentStartDate) OR
(ISNULL(Table.IsCurrent, 0) != 1 AND Table.EndDate = MaxDates.MaxEndDate)
WHERE
table.CurriculumId = 12

Answer 2

Give each row a rank in its curriculumid group, using ROW_NUMBER with an appropriate order by clause. Then only take the records ranked 1 (i.e. best matching).
select
intprofessionalexperienceid,
iscurrent,
startdate,
enddate
from
(
select mytable.*
row_number() over
(
partition by curriculumid
order by
case when iscurrent = 1 then 1 else 2 end,
case when iscurrent = 1 then startdate else enddate end desc
) as rn
from mytable
) ranked
where rn = 1;
(I know this doesn't actually answer your question, but is the straight-forward way to approach the problem in my opinion.)

Answer 3

Try to use CASE statemen this way:
SELECT table.IntProfessionalExperienceId,
table.IsCurrent,
table.StartDate,
table.EndDate
FROM table
WHERE table.CurriculumId = 12
AND table.EndDate = CASE
WHEN table.IsCurrent = 1
THEN (
SELECT max(table.StartDate)
FROM table
WHERE table.IsCurrent = 1
AND table.CurriculumId = 12
GROUP BY table.CurriculumId
)
ELSE
(
SELECT max(table.EndDate)
FROM table
WHERE table.CurriculumId = 12
GROUP BY table.CurriculumId
)
END