Changing datetime value in MySQL - mysql

Question

What I am trying to do, is to set beginning of time interval, if that is not correctly set into stored procedure. However, it somehow does not work very well..
This is my code:
CREATE PROCEDURE intervals_generator (IN start DATETIME, IN ending DATETIME, IN intervalis INT)
BEGIN
-- temp values
DECLARE next_date DATETIME;
-- result temp values
DECLARE start_temp DATETIME;
DECLARE ending_temp DATETIME;
-- date formatting variables
DECLARE year CHAR(20);
DECLARE month CHAR(20);
DECLARE day CHAR(20);
DECLARE new_start CHAR(20);
-- SET starting date if is incorrect DATE_FORMAT(NOW(), '%d %m %Y')
SET year := DATE_FORMAT(start, '%Y');
SET month := DATE_FORMAT(start, '%c');
SET day := DATE_FORMAT(start, '%e');
IF intervalis = '1_day' THEN
BEGIN
SET new_start := year+' '+month+' '+day+' 00:00:00';
END;
ELSEIF intervalis = '1_month' THEN
BEGIN
SET new_start := year+' '+month+' 1 00:00:00';
END;
ELSEIF intervalis = '1_quarter' THEN
BEGIN
IF MONTH(start) IN (2, 3) THEN
SET month := 1;
ELSEIF MONTH(start) IN (5, 6) THEN
SET month := 4;
ELSEIF MONTH(start) IN (8, 9) THEN
SET month := 7;
ELSEIF MONTH(start) IN (11, 12) THEN
SET month := 10;
END IF;
SET new_start := year+' '+month+' 1 00:00:00';
END;
ELSEIF intervalis = '1_year' THEN
BEGIN
SET new_start := year+' 1 1 00:00:00';
END;
END IF;
SET start := STR_TO_DATE(new_start, '%Y %c %e %h:%i:%s');
SELECT year, month, day, start;
DROP TEMPORARY TABLE IF EXISTS intervals_result;
END//
DELIMITER ;
I have tried many different formattings settings and functions, but the output is still wrong, like this:
mysql> CALL intervals_generator('2013-02-01 00:00:00', '2015-12-31 00:00:00', '1_year');
+------+-------+------+---------------------+
| year | month | day | start |
+------+-------+------+---------------------+
| 2013 | 2 | 1 | 2016-00-00 00:00:00 |
+------+-------+------+---------------------+
1 row in set (0.02 sec)
Query OK, 0 rows affected, 1 warning (0.02 sec)
I really dont understand why output is "2016-00-00" instead of "2013-01-01". year, month and day variables are defined as CHAR and also function that extracts them from datetime should be returning CHAR. And function STR_TO_DATE should also been taking CHAR format, so it is a mystery for me.
If anyone has some idea, please give me hint.

Answer 1

If you work in DATEs instead of strings, you can make use of MySQL's date functions and operators and make everything a whole lot simpler... but not too simple because this is MySQL.
The problem with MySQL and dates is its date functionality is a real mish-mash that sometimes works with DATEs, sometimes with strings, sometimes with integers, and is missing basic functionality. It lacks a simple function to set a piece of a date; there's no function to change the MONTH part of a DATE to February. There's not even a good way to make a date from the year, month and day, closest thing you get is MAKEDATE() which takes a year and the day of the year (?!). Fortunately, DATEs in MySQL respond to math operations and it's better than messing with strings.
If you have, for example, 2013-02-12 and want 2013-02-01 you have to first make a new date with just the year using MAKEDATE, then add the month part.
-- 2013-01-01
SET new_date := MAKEDATE(YEAR(old_date), 1);
-- 2013-02-01
-- Since MONTH returns from 1 to 12, you need to take away one.
SET new_date := new_date + (INTERVAL MONTH(old_date) - 1) MONTH;
After chopping out all the unused variables, changing to date math, and using the CASE statement instead of a big IF/ELSE chain, we get this:
CREATE PROCEDURE intervals_generator (IN start_date DATE, IN intervals TEXT)
BEGIN
DECLARE new_start DATE;
CASE intervals
WHEN '1_day' THEN
-- Nothing to do, DATE has already truncated the time portion.
SET new_start := start_date;
WHEN '1_month' THEN
-- Set to the year and month of the start date
SET new_start := MAKEDATE(YEAR(start_date), 1) + INTERVAL (MONTH(start_date) - 1) MONTH;
WHEN '1_quarter' THEN
BEGIN
-- Set to the year and month of the start date
SET new_start := MAKEDATE(YEAR(start_date), 1) + INTERVAL (MONTH(start_date) - 1) MONTH;
-- Subtract the necessary months for the beginning of the quarter
SET new_start := new_start - INTERVAL (MONTH(new_start) - 1) % 3 MONTH;
END;
WHEN '1_year' THEN
-- Set the date to the first day of the year
SET new_start := MAKEDATE(YEAR(start_date), 1);
END CASE;
SELECT new_start;
END//
Try it out.

Answer 2

This statement is not doing what you expect:
SET new_start := year+' '+month+' '+day+' 00:00:00';
In MySQL, the + operator does addition. That's it, not concatenation.
I think you intend:
SET new_start := concat(year, ' ', month, ' ', day, ' 00:00:00');
I haven't looked at the rest of the logic to see if it makes sense, but this is one glaring problem.

Answer 3

Instead of slicing and building new date from parts(concatenating) which is error-prone you can use built-in DATE_FORMAT:
SqlFiddleDemo
SET #date = '2013-05-03 10:05:00';
SELECT CAST(#date AS DATETIME) AS Date,
DATE_FORMAT(#date ,'%Y-01-01 00:00:00') AS Year_Allign,
CASE EXTRACT(QUARTER FROM #date)
WHEN 1 THEN DATE_FORMAT(#date ,'%Y-01-01 00:00:00')
WHEN 2 THEN DATE_FORMAT(#date ,'%Y-04-01 00:00:00')
WHEN 3 THEN DATE_FORMAT(#date ,'%Y-07-01 00:00:00')
WHEN 4 THEN DATE_FORMAT(#date ,'%Y-10-01 00:00:00')
ELSE NULL END AS Quarter_Allign,
DATE_FORMAT(#date ,'%Y-%m-01 00:00:00') AS Month_Allign,
DATE_FORMAT(#date ,'%Y-%m-%d 00:00:00') AS Day_Allign;
SqlFiddleDemo2
SET #date = '2013-05-03 10:05:00';
SET #allign = '1_QUARTER';
SELECT
CAST(#date AS DATETIME) AS Date,
CASE #allign
WHEN '1_YEAR' THEN DATE_FORMAT(#date ,'%Y-01-01 00:00:00')
WHEN '1_QUARTER' THEN (CASE EXTRACT(QUARTER FROM #date)
WHEN 1 THEN DATE_FORMAT(#date ,'%Y-01-01 00:00:00')
WHEN 2 THEN DATE_FORMAT(#date ,'%Y-04-01 00:00:00')
WHEN 3 THEN DATE_FORMAT(#date ,'%Y-07-01 00:00:00')
WHEN 4 THEN DATE_FORMAT(#date ,'%Y-10-01 00:00:00')
ELSE NULL END)
WHEN '1_MONTH' THEN DATE_FORMAT(#date ,'%Y-%m-01 00:00:00')
WHEN '1_DAY' THEN DATE_FORMAT(#date ,'%Y-%m-%d 00:00:00')
ELSE NULL
END AS Alligned;