Give every "empty" age band 0 - mysql

Question

I have a query like the following:
SELECT
COUNT(*) AS `members`,
CASE
WHEN age >= 10 AND age <= 20 THEN '10-20'
WHEN age >=21 AND age <=30 THEN '21-30'
WHEN age >=31 AND age <=40 THEN '31-40'
WHEN age >=41 AND age <= 50 THEN '41-50'
WHEN age >=51 AND age <=60 THEN '51-60'
WHEN age >=61 AND age <=70 THEN '61-70'
WHEN age >= 71 THEN '71+'
END AS ageband
FROM `members`
GROUP BY ageband
Retrieves
How do I populate the empty age bands with 0?
This is what I am looking to achieve from my query above:
members ageband
1 10-20
0 21-30
2 31-40
0 41-50
1 51-60
0 61-70
1 71+
There are NO member in range of 41-50, hence I put 0.

Answer 1

The rows you wish for are not displayed because there's no data for them. Outer join (left or right) all possibilities and they will be displayed:
SELECT
COUNT(members.whatever_column_best_would_be_primary_key) AS `members`,
ages.age_range
FROM `members`
RIGHT JOIN (
SELECT '10-20' as age_range
UNION ALL
SELECT '21-30'
UNION ALL
SELECT '31-40'
UNION ALL
SELECT '41-50'
UNION ALL
SELECT '51-60'
UNION ALL
SELECT '61-70'
UNION ALL
SELECT '71+'
)ages ON ages.age_range = CASE
WHEN members.age >= 10 AND age <= 20 THEN '10-20'
WHEN members.age >=21 AND age <=30 THEN '21-30'
WHEN members.age >=31 AND age <=40 THEN '31-40'
WHEN members.age >=41 AND age <= 50 THEN '41-50'
WHEN members.age >=51 AND age <=60 THEN '51-60'
WHEN members.age >=61 AND age <=70 THEN '61-70'
WHEN members.age >= 71 THEN '71+'
END
GROUP BY ages.age_range

Answer 2

I don't know if I got your question right. Did you miss the part with ELSE in the manual?
SELECT
COUNT(*) AS `members`,
CASE
WHEN age >= 10 AND age <= 20 THEN '10-20'
WHEN age >=21 AND age <=30 THEN '21-30'
WHEN age >=31 AND age <=40 THEN '31-40'
WHEN age >=41 AND age <= 50 THEN '41-50'
WHEN age >=51 AND age <=60 THEN '51-60'
WHEN age >=61 AND age <=70 THEN '61-70'
WHEN age >= 71 THEN '71+'
ELSE '0'
END AS ageband
FROM `members`
GROUP BY ageband

Answer 3

you may use coalsce !
try this
SELECT
COALESCE(COUNT(*),0) AS `members`,
CASE
WHEN age >= 10 AND age <= 20 THEN '10-20'
WHEN age >=21 AND age <=30 THEN '21-30'
WHEN age >=31 AND age <=40 THEN '31-40'
WHEN age >=41 AND age <= 50 THEN '41-50'
WHEN age >=51 AND age <=60 THEN '51-60'
WHEN age >=61 AND age <=70 THEN '61-70'
WHEN age >= 71 THEN '71+'
END AS ageband
FROM `members`
GROUP BY ageband

Answer 4

You can first build a table containing your age bands. Say CREATE TABLE ages(ageband text);:
min_age | max_age | ageband
---------------------------
10 | 20 | 10-20
21 | 30 | 21-30
31 | 40 | 31-40
41 | 50 | 41-50
51 | 60 | 51-60
61 | 70 | 61-70
71 | 99 | 71+
Then you FULL OUTER JOIN:
SELECT
COUNT(members.*) AS `members`,
ageband
FROM
`members`
FULL OUTER JOIN `ages` ON
age BETWEEN min_age AND max_age
GROUP BY ageband;
(You can't use COUNT(*) but have to be more specific and use COUNT(members.*) because there will be rows with an empty ageband and NULL values coming from members and you want to count them as 0. COUNT() counts non-NULL items only.)
(Furthermore, the advantage of FULL OUTER JOIN over RIGHT JOIN is that if you have an age not covered, such as 8, 20.5, 101 or text, it will still be listed with an ageband = NULL so you see the problem.)

Answer 5

even #FancyPants has been provided solution, you can also try below query in different way:
SELECT IFNULL(cnt,0) AS 'members', ages.age_range AS 'ageband' FROM (
SELECT '10-20' AS age_range
UNION ALL
SELECT '21-30'
UNION ALL
SELECT '31-40'
UNION ALL
SELECT '41-50'
UNION ALL
SELECT '51-60'
UNION ALL
SELECT '61-70'
UNION ALL
SELECT '71+') AS ages
LEFT JOIN
(SELECT COUNT(*) AS cnt,
CASE WHEN age>=10 AND age<=20 THEN '10-20'
WHEN age>=21 AND age<=30 THEN '21-30'
WHEN age>=31 AND age<=40 THEN '31-40'
WHEN age>=41 AND age<=50 THEN '41-50'
WHEN age>=51 AND age<=60 THEN '51-60'
WHEN age>=61 AND age<=70 THEN '61-70'
WHEN age>=71 THEN '71+' END age_range
FROM members GROUP BY age_range) a
ON a.age_range=ages.age_range