Compter à partir de 2 tables et grouper par mois

Joignez les deux tables avec le mois :

SELECT MONTH(I.date) AS `month`
     , COUNT(I.ID) AS `countin`
     , COUNT(O.ID) AS `countOUT`
  FROM TableIN I
 LEFT JOIN TableOUT O
    ON MONTH(I.Date) = MONTH(O.Date)
 GROUP BY MONTH(I.date)
UNION
SELECT MONTH(O.date) AS `month`
     , COUNT(I.ID) AS `countin`
     , COUNT(O.ID) AS `countOUT`
  FROM TableIN I
 RIGHT JOIN TableOUT O
    ON MONTH(I.Date) = MONTH(O.Date)
 GROUP BY MONTH(I.date);

Résultat :

| MONTH | COUNTIN | COUNTOUT |
------------------------------
|     5 |       1 |        1 |
|     7 |       1 |        1 |
|     6 |       0 |        1 |

Voir ce SQLFiddle

De plus, pour classer votre résultat par mois, vous devez utiliser une sous-requête comme celle-ci :

SELECT * FROM
(
    SELECT MONTH(I.date) AS `month`
         , COUNT(I.ID) AS `countin`
         , COUNT(O.ID) AS `countOUT`
      FROM TableIN I
     LEFT JOIN TableOUT O
        ON MONTH(I.Date) = MONTH(O.Date)
     GROUP BY MONTH(I.date)
    UNION
    SELECT MONTH(O.date) AS `month`
         , COUNT(I.ID) AS `countin`
         , COUNT(O.ID) AS `countOUT`
      FROM TableIN I
     RIGHT JOIN TableOUT O
        ON MONTH(I.Date) = MONTH(O.Date)
     GROUP BY MONTH(I.date)
    ) tbl
ORDER BY Month;

Voir ce SQLFiddle