Find number of Wednesdays (or other weekday) in a month - Oracle SQL - oracle

Question

I found this query for finding the number of Sundays in a month.
I have been tinkering with it but cannot figure out how to change it to get, say, the number of Wednesdays in a month, for example. Can you show me how?
with
months as (
select add_months(trunc(sysdate,'YEAR'),level-13) month
from dual
connect by level <= 36
)
select to_char(month,'YYYY') year,
to_char(month,'Month') month,
to_char(month,'Day') first_day,
to_char(last_day(month),'Day DD') last_day,
4+
case
when to_char(last_day(month),'DD') - decode(to_char(month,'D'),1,0,8 -to_char(month,'D')) >= 29
then 1
else 0
end nb_sunday
from months

Answer 1

Here's the game: You give me a year (like 2015) and a day of the week, in the form of a three-letter string (like 'Wed'). I will return a table with each month of that year and with the count of days-of-week equal to your input in each month.
Simply implementing here the suggestion from my Comment to MT0's Answer. I am hard-coding the year and the day-of-week (in a CTE) since "how to pass parameters to a query" (through bind variables and such) is not the focus in this thread.
with
inputs ( yr, day_of_week ) as (
select 2015, 'Wed' from dual
),
prep ( dec31 ) as (
select to_date(to_char(yr - 1) || '-12-31', 'yyyy-mm-dd') from inputs
)
select to_char(add_months(dec31, level), 'Mon-yyyy') as mth,
( next_day(add_months(dec31, level) , day_of_week) -
next_day(add_months(dec31, level - 1), day_of_week) ) / 7 as cnt
from inputs cross join prep
connect by level <= 12;
MTH CNT
-------- ----
Jan-2015 4
Feb-2015 4
Mar-2015 4
Apr-2015 5
May-2015 4
Jun-2015 4
Jul-2015 5
Aug-2015 4
Sep-2015 5
Oct-2015 4
Nov-2015 4
Dec-2015 5
12 rows selected.

Answer 2

The last wednesday of the month is given by:
TRUNC( NEXT_DAY( LAST_DAY( :month ) - INTERVAL '7' DAY, 'WEDNESDAY' ) )
The first wednesday of the month is given by:
NEXT_DAY( TRUNC( :month, 'MM' ) - INTERVAL '1' DAY, 'WEDNESDAY' )
Subtracting gives the number of days between them. Divide by 7 and add 1 and you get the number of Wednesdays:
SELECT ( TRUNC( NEXT_DAY( LAST_DAY( :month ) - INTERVAL '7' DAY, 'WEDNESDAY' ) )
- NEXT_DAY( TRUNC( :month, 'MM' ) - INTERVAL '1' DAY, 'WEDNESDAY' )
) / 7 + 1
AS number_of_wednesdays
FROM DUAL;
Or you can just use the difference between the first Wednesday of the month and of the next month as suggested by #mathguy
SELECT ( NEXT_DAY(
ADD_MONTHS( TRUNC( :month, 'MM' ), 1 ) - INTERVAL '1' DAY,
'WEDNESDAY'
)
- NEXT_DAY(
TRUNC( :month, 'MM' ) - INTERVAL '1' DAY,
'WEDNESDAY'
)
) / 7
AS number_of_wednesdays
FROM DUAL;