Mysql case when and order by issue - mysql

Question

Have this query:
SELECT
count(*) as Total,
SUM(CASE WHEN gender = 1 then 1 ELSE 0 END) Male,
SUM(CASE WHEN gender = 2 then 1 ELSE 0 END) Female,
SUM(CASE WHEN gender = 0 then 1 ELSE 0 END) Unknown,
CASE
WHEN age>2 AND age<15 THEN '2-15'
WHEN age>18 AND age<25 THEN '18-25'
END AS var
FROM
persons
WHERE
1=1
AND `date` > '2012-01-10'
AND `date` < '2013-01-07'
GROUP BY
CASE
WHEN age>2 AND age<15 THEN '2-15'
WHEN age>18 AND age<25 THEN '18-25'
END
And is resulting this:
Total Male Female Unknown var
29 17 12 0 NULL
7 0 7 0 18-25
3 0 3 0 2-15
1st question: Why is this resulting that NULL ? What could be done to only show results with values?
2nd question: mysql is ordering my var column with 18-25 before 2-15, migth be because of number 1 cames first then number 2. But the point is order that as numbers, and 2 came first then 18.
Cheers :)

Answer 1

1st answer:
It is NULL because it does not satisfy any of your CASE conditions for the age. Adding a clause to the WHERE like this should do it:
WHERE (age > 2 AND age < 15) OR (age > 18 AND age < 25)
2nd answer:
You are correct, it is ordering them by strings (because that is what they are). Just change the direction of the sort by doing ORDER ASC or ORDER DESC

Answer 2

This is because all CASE expression has an (implied, default) ELSE NULL part. SO, any age value that is not caught by either the age>2 AND age<15or the age>18 AND age<25 condition, results in the NULL value being grouped.
Solution is to add one more restriction at the WHERE clause:
WHERE 1=1
AND `date` > '2012-01-10' AND `date` < '2013-01-07'
AND ( (age>2 AND age<15) OR (age>18 AND age<25) ) -- this
For the second question, you can use a function on age to avoid the comparison being made on the var (which is a string):
ORDER BY MIN(age)
or just:
ORDER BY age
None of the above is by the SQL standard but it works in MySQL, under the default non-ANSI settings. If you want to be 100% by the book, you can change slightly the var:
SELECT count(*) as Total,
SUM(CASE WHEN gender = 1 then 1 ELSE 0 END) Male,
SUM(CASE WHEN gender = 2 then 1 ELSE 0 END) Female,
SUM(CASE WHEN gender = 0 then 1 ELSE 0 END) Unknown,
CASE
WHEN age>2 AND age<15 THEN '02-15' -- this was changed
WHEN age>18 AND age<25 THEN '18-25'
END AS var
FROM persons
WHERE 1=1
AND `date` > '2012-01-10' AND `date` < '2013-01-07'
AND ( (age>2 AND age<15) OR (age>18 AND age<25) )
GROUP BY
CASE
WHEN age>2 AND age<15 THEN '02-15'
WHEN age>18 AND age<25 THEN '18-25'
END
ORDER BY var ;

Answer 3

you are getting NULL
because it doesnt meet your CASE
CASE
WHEN age>2 AND age<15 THEN '2-15' // U HAVE BETWEEN 2-15
WHEN age>18 AND age<25 THEN '18-25' // u have between 18-25
// but u dont have between 15-18
//and u get null because your value is between 15-18
so try to add other case in that range.
second question because they are strings , not numbers.
try order them by age