Why are these NVL() statements wrong in SQL? - sql

Question

SELECT inv_no,NVL2(inv_amt,inv_date,'Not Available')
FROM invoice;
SELECT inv_no,NVL2(inv_amt,inv_amt*.25,'Not Available') FROM invoice;
getting
ORA-01858: a non-numeric character was found where a numeric was expected
ORA-01722: invalid number
respectively
Please suggest what are the datatypes I can give in expr1 and expr2.
Also Please tell me how this is right?
SELECT inv_no,NVL2(inv_date,sysdate-inv_date,sysdate)
FROM invoice

Answer 1

The 2nd and 3rd parameters to NVL2 should be the same datatype. So, assuming inv_date is a DATE, you'd need to have that as a varchar2 like;
SELECT inv_no, NVL2(inv_amt, to_char(inv_date, 'dd-mon-yyyy hh24:mi:ss'), 'Not Available') FROM ...
or whatever format string you wanted. Otherwise Oracle will convert the 3rd parameter 'Not Available' to match the 2nd parameter's data type. This will try to convert 'Not Available' to a date and crash.
Similarly in the 2nd example, you have to convert the inv_amt*.25 to a char viato_char, e.g. to_char(inv_amt*.25).

Answer 2

Your first 2 examples attempt to have a date / numeric and text results for the same field. This will cause an error when Oracle attempts to convert this text to either types. You'll need to use the to_char(field) function on the date / numeric fields to convert them to text.
Lastly a date is in fact a number added to a databases base date. For example a date is a number of base day and the decimal it has is the ratio of a day, e.g. 0.5 is 12 hours, and the database has a base date, e.g. 01-Jan-1900 or 01-Jan-2000. This is why when you do date - date the result is a number and a date can also be represented as a number.

Answer 3

Refer to: https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions106.htm
The 2nd argument should be either character or numeric. The 2nd argument gives datatype to which the 3rd argument will be implictly converted. So:
1. The first one SELECT inv_no,NVL2(inv_amt,inv_date,'Not Available') FROM invoice; is incorrect as you cannot have DATE as a 2nd argument.
The same way:
SELECT NVL2(NULLIF(1, 1), SYSDATE, 'A') FROM DUAL; -- FAILS;
SELECT NVL2(NULLIF(1, 0), SYSDATE, 'A') FROM DUAL; -- FAILS;
SELECT NVL2(NULLIF(1, 1), 'A', SYSDATE) FROM DUAL; -- PASS; returns SYSDATE (as char datatype)
SELECT NVL2(NULLIF(1, 0), 'A', SYSDATE) FROM DUAL; -- PASS; returns 'A';
2. The second is invalid too: SELECT inv_no,NVL2(inv_amt,inv_amt*.25,'Not Available') FROM invoice; Oracle tries to implicitly convert 3rd argument to the datatype of the 2nd arugment, meaning it tries to convert 'Not available' to numeric value.
It will work when you swap 2nd and 3rd argument:
SELECT inv_no,NVL2(inv_amt,'Not Available',inv_amt*.25) FROM invoice;
inv_amt*.25 will be implicitly converted to character data; This example is useless as it will get you NULL*.25 making it NULL.
What you can do however is you can make both parameters as characters by converting numeric result to character:
SELECT inv_no,NVL2(inv_amt,TO_CHAR(inv_amt*.25), 'Not Available') FROM invoice;
It will make some sense particularly when you want to display currency.
3. The third one is more complex
The only way I can explain is that the 2nd argument gets converted to character before dealing with the third one. That way the third one will get converted to characters too. This would make sense given the two cases:
SELECT NVL2(2, SYSDATE - TO_DATE('10-JAN-83'), SYSDATE) from dual; -- PASSES
SELECT NVL2(2, 2.2, SYSDATE) from dual; -- FAILS.
Finally: I would advise to stick to Oracle's documentation and use explicit conversion.