Converting Decode to Case

Question

I'm a beginner with Oracle SQL and I have a select statement with the following code:
max(decode(EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD, '1000', EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE, '2000', EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE, '4000', EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE))
I've been asked to create several more columns in a similar manner. My concern is that since this relies on 'decode' which is less efficient than 'case', that adding more columns using this approach will bog down an already not efficient query. Bottom line my problem is that I don't really understand this 'decode' since all the explanations of 'decode' I've found stop at four parameters.
Could someone please show me how the expression above looks in If-then-else terms as well as comment on how to convert this 'decode' to 'case'? Thanks in advance!

Answer 1

1) The if-then-else stmt
-- Longest way
if EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD = '1000' then
return value of EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE;
elsif EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD = '2000' then
return value of EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE;
elsif EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD = '4000' then
return value of EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE;
else return null;
end if;
-- shorter way
if EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD = '1000' or
EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD = '2000' or
EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD = '4000' then
return value of EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE;
else return null;
end if;
-- shortest if-then-else code
if EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD in ('1000', '2000', '4000') then
return value of EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE;
else return null;
end if;
2) The MAX() function
The MAX function will return the greatest value of EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE in step 1 above.
2) Using CASE
select
max(case when (EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_CD in('1000','2000','4000'))
then EDW.V_RECRUIT_TEST_RESULT_HIST.TEST_RESULT_TEST_SCORE end)
into ...your_item_or_local_variable
from EDW.V_RECRUIT_TEST_RESULT_HIST;
Try the code above, good luck
v/r
Vien Tran