Converting a nested decode into equivalent CASE statement (needed for conversion from Oracle to PostgreSQL) - sql

Question

I am on Oracle 11.2.0.4. I have a nested DECODE statement that I need to convert into CASE statement. Can someone help on it. I am not sure of how it is done and in fact I don't fully understand the logic of it. If someone can explain what it basically intends to do and what would be the rewritten function using CASE that is Very useful to me. Here is the function...(note: do not worry about table joins , there are 3 tables and one condition etc. please focus on the DECODE and its conversion to CASE). Also I must manually convert and tools are not an option.
CREATE OR REPLACE FUNCTION TMP_Func
RETURN NUMBER
IS
V NUMBER;
BEGIN
SELECT DECODE (
NVL (tab1.flag1, '~'),
'D', DECODE (tab1.code_oid, NVL (tab3.bu_id, '-'), 1, 0),
'C', DECODE (tab1.code_oid, NVL (tab3.cost_id, '-'), 1, 0),
DECODE (tab2.oid,
DECODE (tab1.co_id, NULL, tab2.oid, tab1.co_id), 1,
0))
INTO V
FROM tab1, tab2, tab3
WHERE tab2.OID = tab1.sec_id;
RETURN V;
END;

Answer 1

something like this :
select CASE WHEN COALESCE(tab1.flag1,'~') = 'D' THEN
CASE WHEN tab1.code_oid=COALESCE(tab3.bu_id, '-') THEN 1 else 0 end
WHEN COALESCE(tab1.flag1,'~')='C' THEN
CASE WHEN tab1.code_oid=COALESCE(tab3.cost_id, '-') THEN 1 else 0 end
else
CASE WHEN tab2.oid=COALESCE(tab1.co_id,tab2.oid) THEN 1 else 0 end
end
FROM tab1, tab2, tab3
WHERE tab2.OID = tab1.sec_id;
NVL is replaced by COALESCE
DECODE(a,b,c,d,e,...,f) is replaced by :
CASE WHEN a=b THEN c
WHEN a=d THEN e
...
else f
end
your last decode ( DECODE (tab1.co_id, NULL, tab2.oid, tab1.co_id), 1,
0))) is in fact an NVL( tab1.co_id,tab2.oid)

Answer 2

That would be something like this:
SELECT CASE
WHEN NVL (tab1.flag1, '~') = 'D'
THEN
CASE
WHEN tab1.code_oid = NVL (tab3.bu_id, '-') THEN 1
ELSE 0
END
WHEN NVL (tab1.flag1, '~') = 'C'
THEN
CASE
WHEN tab1.code_oid = NVL (tab3.cost_id, '-') THEN 1
ELSE 0
END
ELSE
CASE
WHEN tab2.oid =
CASE
WHEN tab1.co_id IS NULL THEN tab2.oid
ELSE tab1.co_id
END
THEN
1
ELSE
0
END
END
INTO v
FROM ...
Note that your FROM clause contains 3 tables, but WHERE joins just two of them. What about tab3? Also, consider switching to ANSI join.

Answer 3

The following should do what you want:
SELECT CASE COALESCE(TAB1.FLAG1, '~')
WHEN 'D' THEN CASE
WHEN TAB1.CODE_OID = COALESCE(TAB3.BU_ID, '-') THEN 1
ELSE 0
END
WHEN 'C' THEN CASE
WHEN TAB1.CODE_OID = COALESCE(TAB3.COST_ID, '~') THEN 1
ELSE 0
END
ELSE CASE
WHEN TAB2.OID = COALESCE(TAB1.CO_ID, TAB2.OID) THEN 1
ELSE 0
END
END
INTO V
FROM TAB1
INNER JOIN TAB2
ON TAB2.OID = TAB2.SEC_ID
CROSS JOIN TAB3;
Note that NVL and COALESCE are slightly different in a couple of respects, although neither seems to be a factor here. First, NVL always takes two arguments, while COALESCE can take as many as you care to supply - it returns the first non-NULL argument. Second, NVL always evaluates both of its arguments (e.g. if a function is specified for one or both of the arguments to NVL, both functions are called), while COALESCE only evaluates as many of the arguments are necessary to find a non-NULL result; thus, if COALESCE is given two functions for arguments and the first returns a non-NULL value, the second function is never called. Not an issue here, but perhaps important (due to side effects) in other cases.
Best of luck.