How to to count numbers in consecutive month in SQL oracle

Question

I am learning advance sql in oracle. I want to find the count of numbers in two consecutive month. In normal sql my result could be produced like this way

select count(distinct number) 
from table1 
where date='1-sep-21' 
and number in (select distinct number 
               from table2 
               where date = '1-aug-21')

So these numbers from September are also found in August. What I want to get the result like

Month   count_number(current_month but also in previous month)
08-2021         122377 
09-2021          40000 
10-2021.          230

I am working in Oracle SQL. How can I achieve this? Kindly help me to get the idea.

Answer 1

Your WHERE clause has three problems:

where date='1-sep-21'

First, 'date' is a reserved word, so your column should not be named that. Since you specified that in lower-case and without double-quotes, it would actually be considered case-insenstive and thus collide with the reserved word 'DATE', so I mus assume that is not really the name of the column.

Second, assuming your column is - as it should be - of type DATE, then you are comparing a DATE with the character string '1-sep-21'. This will force an implied TO_DATE to convert that string to a DATE and may or may not throw an error, depending on the setting of NLS_DATE_FORMAT. Bottom line, you should never rely on implied conversions.

Third, you only specify 2 digits for the year of your 'date'. C'mon, man. In 1998-1999, I and millions of my colleagues spent way too much of our time remediating Y2k. And now you try to recreate the problem.