MySQL 5.6 Selecting months and days - mysql

Question

I am struggling to understand the date functions of MySQL.
For example, I want to get all records of this month and the previous month.
From what I understand I'm supposed to use MONTH(NOW()) which returns the integer of the current month and MONTH(NOW())-1 for the previous month. Except when asking for a date in January, this will obviously will not work as it's performed on the result of the function instead of wrapping around and going to December of the previous year.
How would I go about writing a generic function that would work on January too? Since months have varying lengths it wouldn't be possible to subtract a month in milliseconds from NOW() I recon.
For example, this query works fine except on January:
SELECT
SUM(CASE WHEN MONTH(table_date) = MONTH(NOW())-1 THEN 1 ELSE 0 END) AS LAST_MONTH,
SUM(CASE WHEN MONTH(table_date) = MONTH(NOW()) THEN 1 ELSE 0 END) AS THIS_MONTH
FROM table
WHERE table_date BETWEEN Date_add(NOW(), interval - 1 month) AND NOW()
A similar issue arises with DAY() function since months have varying lengths and writing statements for each and every day doesn't seem very efficient.
What is a more efficient way of selecting the records for each day of any month of interest, regardless of whether that specific day actually has records?

Answer 1

You are vulnerable to pulling records from all years not only the current one stored within table_date. To get rid of it you need to operate on the year as well which can be done by always calculating on the proper date format YYYY-MM-DD not to lose precision.
Here are some calculations used to build up final query.
First day of month:
SELECT DATE_FORMAT(table_date, '%Y-%m-01') FROM table
First day of previous month:
SELECT DATE_FORMAT(DATE_SUB(table_date, INTERVAL 1 MONTH), '%Y-%m-01') FROM table
Last day of month:
SELECT LAST_DAY(table_date) FROM table
Last day of previous month:
SELECT LAST_DAY(DATE_SUB(table_date, INTERVAL 1 MONTH)) FROM table
Your final query:
SELECT
SUM(CASE WHEN table_date BETWEEN DATE_FORMAT(table_date, '%Y-%m-01') AND LAST_DAY(table_date) THEN 1 END) AS this_month,
SUM(CASE WHEN table_date BETWEEN DATE_FORMAT(DATE_SUB(table_date, INTERVAL 1 MONTH), '%Y-%m-01') AND LAST_DAY(DATE_SUB(table_date, INTERVAL 1 MONTH)) THEN 1 END) AS last_month
FROM table
WHERE
table_date BETWEEN
DATE_FORMAT(DATE_SUB(table_date, INTERVAL 1 MONTH), '%Y-%m-01')
AND LAST_DAY(table_date)
Note: In the case expression I've omitted ELSE 0 since default statement for ELSE in CASE is null and SUM() ignores null values :-)

Answer 2

Your logic is not safe also for dates from other years.
The way to achieve what you want might be to truncate the dates to the first of the month, e.g. (for replaced NOW with 2013-01-20)
select table_date,
case when date_format(table_date, '%Y-%m-01')
= date_format('2013-01-20', '%Y-%m-01') then 1 end AS THIS_MONTH,
case when date_format(table_date, '%Y-%m-01')
= date_add(date_format('2013-01-20', '%Y-%m-01'), interval -1 month) then 1 end AS LAST_MONTH
from x;
See this SqlFiddle

Answer 3

Your logic converts the date into integer then subtracts 1 from it. For days and month like January, the value becomes 1. From which if you subtract 1, it becomes zero, which is not a valid date or month.
For month try this: sum(case when month(table_date) = month(date_sub(curdate(), interval 1 month)) then 1 else 0 end) as last_month.
You can use similar logic for days

Answer 4

Can you please try with this :
SELECT table_date,
SUM(CASE WHEN (LEFT(table_date, 7) = LEFT(NOW(),7)) THEN
1
ELSE 0 END) AS THIS_MONTH,
SUM(CASE WHEN
(LEFT(table_date, 7) = LEFT(DATE_SUB(NOW(), INTERVAL 1 MONTH),7)) THEN
1 ELSE 0 END) AS LAST_MONTH
FROM table
WHERE LEFT(table_date, 7) BETWEEN LEFT(DATE_SUB(NOW(), INTERVAL 1 MONTH),7)
AND LEFT(NOW(),7)