GROUP BY MONTH() hide result - mysql

Question

I'm trying to count how many result in each month.
This is my query :
SELECT
COUNT(*) as nb,
CONCAT(MONTH(t.date),0x3a,YEAR(t.date)) as period
FROM table1 t
WHERE t.criteria = 'value'
GROUP BY MONTH(t.date)
ORDER BY YEAR(t.date)
My Result:
nb period
---------------
7 6:2009
46 8:2009
2 10:2009
1 11:2009
14 1:2009
9 9:2010
161 7:2010
5 2:2010
88 3:2010
28 4:2010
4 5:2011
2 12:2011
The problem is, I'm sure that I've result between 5:2011 & 12:2011 , and each other period
since 2009 ... :/
This is a problem of my request or mysql configuration ?
Thx a lot

Answer 1

You have to group by both the year and the month. Otherwise your April 2012 rows are grouped with April 2011 (and April 2010 ...) rows as well.
SELECT
COUNT(*) AS nb,
CONCAT(MONTH(t.date), ':', YEAR(t.date)) AS period
FROM table1 AS t
WHERE t.criteria = 'value'
GROUP BY YEAR(t.date)
, MONTH(t.date) ;
(and is there a reason you used 0x3a and not ':'?)
You could also use some other DATE and TIME functions of MySQL so there are fewer functions calls per row and probably a more efficient query:
SELECT
COUNT(*) AS nb,
DATE_FORMAT(t.date, '%m:%Y') AS period
FROM table1 AS t
WHERE t.criteria = 'value'
GROUP BY EXTRACT( YEAR_MONTH FROM t.date) ;
For several queries, it's useful to have a permanent Calendar table in your database (with all dates or all year-months) or even several Calendar tables. Example:
CREATE TABLE CalendarYear
( Year SMALLINT UNSIGNED NOT NULL
, PRIMARY KEY (Year)
) ENGINE = InnoDB ;
INSERT INTO CalendarYear
(Year)
VALUES
(1900), (1901), ..., (2099) ;
CREATE TABLE CalendarMonth
( Month TINYINT UNSIGNED NOT NULL
, PRIMARY KEY (Month)
) ENGINE = InnoDB ;
INSERT INTO CalendarMonth
(Month)
VALUES
(1), (2), ..., (12) ;
Those can also help us make the one we'll need here:
CREATE TABLE CalendarYearMonth
( Year SMALLINT UNSIGNED NOT NULL
, Month TINYINT UNSIGNED NOT NULL
, FirstDay DATE NOT NULL
, NextMonth_FirstDay DATE NOT NULL
, PRIMARY KEY (Year, Month)
) ENGINE = InnoDB ;
INSERT INTO CalendarYearMonth
(Year, Month, FirstDay, NextMonth_FirstDay)
SELECT
y.Year
, m.Month
, MAKEDATE(y.Year, 1) + INTERVAL (m.Month-1) MONTH
, MAKEDATE(y.Year, 1) + INTERVAL (m.Month) MONTH
FROM
CalendarYear AS y
CROSS JOIN
CalendarMonth AS m ;
Then you can use the Calendar tables to write more complex queries, like the variation you want (with missing months) and probably more efficiently. Tested in SQL-Fiddle:
SELECT
COUNT(t.date) AS nb,
CONCAT(cal.Month, ':', cal.Year) AS period
FROM
CalendarYearMonth AS cal
JOIN
( SELECT
YEAR(MIN(date)) AS min_year
, MONTH(MIN(date)) AS min_month
, YEAR(MAX(date)) AS max_year
, MONTH(MAX(date)) AS max_month
FROM table1
WHERE criteria = 'value'
) AS mm
ON (cal.Year, cal.Month) >= (mm.min_year, mm.min_month)
AND (cal.Year, cal.Month) <= (mm.max_year, mm.max_month)
LEFT JOIN
table1 AS t
ON t.criteria = 'value'
AND t.date >= cal.FirstDay
AND t.date < cal.NextMonth_FirstDay
GROUP BY
cal.Year, cal.Month ;

Answer 2

You must also GROUP BY the year:
GROUP BY MONTH(t.date), YEAR(t.date)
Your original query uses YEAR(t.date) in the SELECT clause outside of any aggregate function without grouping by it -- as a result, you get exactly 12 groups (one for each possible month) and for each group (that possibly contains dates across many years) a "random" year is chosen by MySql for selection. Strictly speaking, this is meaningless and the query should never have been allowed to execute. But MySql... sigh.