How can I refactor this query - sql

Question

How can I rewrite this SQL code? I would like to avoid the repetitive execution of the case for every record.
SELECT chcr.chsid,
CASE
WHEN EXISTS
(SELECT 1
FROM hdr_run chre,
clmerr ce
WHERE chre.chsid = chcr.chsid
AND chre.run_nmbr < chcr.last_run_nmbr
AND chre.clm_error_sid = ce.clm_error_sid
GROUP BY chre.chsid
HAVING COUNT(chre.clm_error_sid) > 0
)
THEN 'Appended'
ELSE 'Never Appended'
END Run_Detail
FROM
clm_res chcr,
clm_der chde
WHERE chde.chsid = chcr.chsid

Answer 1

Have a look at that query please:
SELECT
chcr.chsid,
CASE
WHEN ce.clm_error_sid IS NOT NULL AND COUNT(chre.clm_error_sid, 0) > 0
THEN 'Appended'
ELSE 'Never Appended'
END Run_Detail
FROM
clm_res chcr
JOIN clm_der chde ON chde.chsid = chcr.chsid
LEFT JOIN hdr_run chre ON chre.chsid = chcr.chsid AND chre.run_nmbr < chcr.last_run_nmbr
LEFT JOIN clmerr ce ON chre.clm_error_sid = ce.clm_error_sid
GROUP BY chcr.chsid
If there's more than one clm_der for each clm_res based on their chsid, we'd have to double check that the COUNT is not counting the extras from clm_der.

Answer 2

I have no data to test this on so all I can go on is the SQL you've provided but from a brief look it does not appear that the GROUP BY and HAVING COUNT() > 0 statements are required as the combination of the INNER JOIN condition in the sub-query and the use of EXISTS in the outer query does the same thing.
Does this have exactly the same functionality:
SELECT chcr.chsid,
CASE
WHEN EXISTS
( SELECT 1
FROM hdr_run chre
INNER JOIN
clmerr ce
ON (chre.clm_error_sid = ce.clm_error_sid)
WHERE chre.chsid = chcr.chsid
AND chre.run_nmbr < chcr.last_run_nmbr
)
THEN 'Appended'
ELSE 'Never Appended'
END Run_Detail
FROM clm_res chcr
INNER JOIN
clm_der chde
ON ( chde.chsid = chcr.chsid );
SQLFIDDLE
Also, repeated execution of the CASE statement is not a bad thing as the optimizer should not duplicate executions of the sub-query.