SQL language programming using decode in SELECT statement - sql

Question

I have two parameters called START_DATE and END_DATE.
If the start date and end date are entered , i would like to use the TO_CHAR to convert to a different format and display ":start_date - :end_date" both in the format 'MON DD, YYYY')
However, if the parameters are not entered i would like to use the TO_CHAR function to display information for the previous month i.e ( MAR 01, 2017- MAR 31,2017).
Here's the code below. Can someone identify my error and assist?
SELECT DECODE (TO_CHAR(:START_DATE, 'MON DD, YYYY',(trunc(trunc(sysdate,'MONTH')-1,'MONTH'))),'MON DD, YYYY')
|| ' - ' || DECODE (TO_CHAR(:END_DATE, 'MON DD, YYYY',(trunc(sysdate,'MONTH')-1)'MON DD, YYYY') BILLPERIOD
How do i write this in sql using the decode function?

Answer 1

I highly recommend you use a CASE statement instead. More control, more readable, cleaner, etc. In my humble opinion of course.
SELECT
CASE
-- If one or both dates are NULL, revert to last month
WHEN start_date IS NULL OR end_date IS NULL THEN
to_char(trunc(trunc(sysdate,'MM')-1,'MM'), 'MON DD, YYYY') || ' - ' || to_char(trunc(sysdate,'MM')-1, 'MON DD, YYYY')
WHEN start_date IS NOT NULL AND end_date IS NOT NULL AND
(start_date < end_date) THEN
to_char(start_date, 'MON DD, YYYY') || ' - ' || to_char(end_date, 'MON DD, YYYY')
-- ** ALWAYS EXPECT THE UNEXPECTED! **
-- Like start_date >= end_date
ELSE '** DATE ERROR **'
END as billperiod
FROM dual;
EDIT: Added a check for start_date must be < end_date.

Answer 2

If you can use CASE in your project try:
SELECT
CASE START_DATE
WHEN NULL THEN TO_CHAR(PREVIOUS_DATE, 'MON DD, YYYY')
ELSE TO_CHAR(START_DATE, 'MON DD, YYYY')
END || ','
CASE END_DATE
WHEN NULL THEN TO_CHAR(PREVIOUS_DATE, 'MON DD, YYYY')
ELSE TO_CHAR(END_DATE, 'MON DD, YYYY')
END "Formated Date"
FROM BILLPERIOD;