Dynamic SubSelects in SQL Select Statement - sql

Question

I am querying a table for some basic information, file number, case type, status, etc. In addition I need a column for every single one of 138 case status types that will display the date the case had that status. Here is a sample:
SELECT FileNum,
CaseType,
CurrentCaseStatus,
(SELECT TOP 1 EventDt FROM caseStatusHistory WHERE CaseID = c.caseID AND CaseStatus = 'CS001' ORDER BY EventDt DESC) AS [Charge - Phone],
(SELECT TOP 1 EventDt FROM caseStatusHistory WHERE CaseID = c.caseID AND CaseStatus = 'CS002' ORDER BY EventDt DESC) AS [Charge - Written],
-- 136 more just like the live above
FROM Case c
I can query another table for all the case status types:
SELECT Code, Description
FROM caseStatus
WHERE Code BETWEEN 'CS001' AND 'CS138'
ORDER BY Code
How can I dynamically create each of those columns instead of having to manually write 138 select statements?

Answer 1

That's going to be terribly slow -- 138 correlated subqueries. I think you can achieve the same result with an OUTER JOIN and a GROUP BY with MAX and CASE:
Select c.filenum,
c.casetype,
c.currentcasestatus,
max(case when csh.CaseStatus = 'CS001' then EventDt end) as [Charge - Phone],
max(case when csh.CaseStatus = 'CS002' then EventDt end) as [Charge - Written]
from case c
left join casestatushistory csh on c.caseid = csh.caseid
group by c.filenum,
c.casetype,
c.currentcasestatus
BTW, I would suggest just writing the statement out -- it won't take that long and it will out perform a dynamic sql approach. I'm not completely sure how you'd get your column names with dynamic sql either unless Phone and Written are in another column.

Answer 2

Try using a PIVOT. The SQL below should work -
--Select the pivot data into a temp table
SELECT c.caseID,
c.FileNum,
c.CaseType,
c.CurrentCaseStatus,
csh.EventDt,
cs.Description
INTO #StatusDates
FROM [Case] c
LEFT JOIN caseStatusHistory csh
ON csh.caseID = c.caseID
LEFT JOIN caseStatus cs
ON cs.Code = csh.CaseStatus
--From the pivot data, get the list of field names (assumes description field is the source for the field name)
DECLARE #statusDescriptions VARCHAR(MAX)
SET #statusDescriptions = ''
SELECT #statusDescriptions = COALESCE(#statusDescriptions+'[','') + Description
FROM (
SELECT DISTINCT Description
FROM #StatusDates
WHERE Description IS NOT NULL
) x
SET #statusDescriptions = REPLACE(#statusDescriptions, '[', '],[') + ']'
SET #statusDescriptions = SUBSTRING(#statusDescriptions, 3, LEN(#statusDescriptions))
--Create a SQL statement to pivot the data into the fields.
DECLARE #sql VARCHAR(MAX)
SET #SQL = '
SELECT *
FROM #StatusDates
PIVOT(MIN(EventDt)
FOR Description IN (' + #statusDescriptions + '))
AS PVTTable '
PRINT #sql
EXEC(#sql)
DROP TABLE #StatusDates