MySQL Add missing Months in resultset - mysql

Question

I'm trying to add missing months in this result set.
Where month is missing, add it with the value 0 for Quantita.
SELECT MONTH(Data) AS Mese,Count(*) AS Quantita
FROM prenotazioni
WHERE Cancellata IS NULL
AND FKCampo = 1
AND YEAR(Data) = YEAR(CURDATE()) -1
GROUP BY Mese
ORDER BY Mese ASC
+------+----------+
| Mese | Quantita |
+------+----------+
| 4 | 123 |
+------+----------+
| 5 | 100 |
+------+----------+
| 7 | 377 |
+------+----------+
| 9 | 54 |
+------+----------+

Answer 1

The following is messy and I am sure there has to be a cleaner way...
So, first I create a table with all the months:
CREATE TABLE all_months (
month_num INT
);
INSERT INTO all_months VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12)
Then I join your query with this table (in my case on the right) and use a case-when to decide if there is a count or not:
SELECT
all_months.month_num,
CASE
WHEN tmp.Quantita is NULL THEN 0
ELSE tmp.Quantita
END as Quantita
FROM (
SELECT MONTH(Data) AS Mese,Count(*) AS Quantita
FROM prenotazioni
WHERE Cancellata IS NULL
AND FKCampo = 1
AND YEAR(Data) = YEAR(CURDATE()) -1
GROUP BY Mese
) as tmp
RIGHT JOIN all_months on all_months.month_num=tmp.Mese
ORDER BY all_months.month_num ASC
Your query is now in tmp and ordering is happening after that. The result is:
month_num Quantita
1 1
2 2
3 0
4 0
5 0
6 0
7 0
8 3
9 1
10 0
11 1
12 1
Demo: here

Answer 2

A solution that does not require creating a new table.
SELECT
`months`.`number`,
CASE
WHEN tmp.Quantita IS NULL THEN 0
ELSE tmp.Quantita
END AS Quantita
FROM (
SELECT MONTH(Data) AS Mese, COUNT(*) AS Quantita
FROM prenotazioni
WHERE Cancellata IS NULL
AND FKCampo = 1
AND YEAR(Data) = YEAR(CURDATE()) -1
GROUP BY Mese
) AS tmp RIGHT JOIN (
SELECT 1 AS `number`
UNION SELECT 2
UNION SELECT 3
UNION SELECT 4
UNION SELECT 5
UNION SELECT 6
UNION SELECT 7
UNION SELECT 8
UNION SELECT 9
UNION SELECT 10
UNION SELECT 11
UNION SELECT 12
) AS `months` ON `months`.`number` = tmp.Mese
ORDER BY `months`.`number` ASC;

Answer 3

One method is to construct or find a table that has all the months you want and use a LEFT JOIN.
In your case, though, conditional aggregation might do this with a simpler query. This assumes that you have at least one row for each month, even if that is filtered out by the existing where clause.
So, this might work:
SELECT MONTH(Data) AS Mese,
SUM(Cancellata IS NULL AND FKCampo = 1 ) AS Quantita
FROM prenotazioni
WHERE YEAR(Data) = YEAR(CURDATE()) - 1
GROUP BY Mese
ORDER BY Mese ASC ;
Of course, if the original table does not have all the months, then they still won't be in the result set and you are back to having to use an outer join.

Answer 4

Same idea of #urban but simplifying the query using left join:
create table prenotazioni
(
id int auto_increment,
Cancellata int default null null,
FKCampo int default 1 null,
Data date default null null,
constraint table_nametes_pk
primary key (id)
);
insert into prenotazioni (Cancellata, FKCampo, Data)
values (null, 1, "2019-04-01"),
(null, 1, "2019-04-01"),
(null, 1, "2019-05-01"),
(null, 1, "2019-05-01"),
(null, 1, "2019-05-01"),
(null, 1, "2019-01-01"),
(null, 1, "2019-01-01"),
(null, 1, "2020-01-01"),
(null, 1, "2018-01-01");
CREATE TABLE all_months (
month_num INT
);
INSERT INTO all_months VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12);
select month_num, count(p.Data)
from
all_months am left join prenotazioni p on (am.month_num = month(p.Data))
where
p.id is null
or
(Cancellata IS NULL AND FKCampo = 1 AND YEAR(Data) = YEAR(CURDATE()) - 1)
GROUP BY am.month_num
ORDER BY am.month_num;
result: