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 ;