MySQL在基于性别和生日的选择情况下显示空值

Question

I have SQL syntax with CASE condition based on birth and gender. Below is the sample query that I use

SELECT 
  Age,
  SUM(gender = 'M') M,
  SUM(gender = 'F') F,
  COUNT(*) TotalPerson 
FROM
  (SELECT 
    CASE
      WHEN FLOOR(DATEDIFF(CURDATE(), birdth) / 365)  60 THEN '60 Above' 
    END Age,
    gender 
  FROM
    society) ageList 
GROUP BY Age

the result become like this:

Age         | M     | F     | TotalPerson
--------------------------------
19 - 25     | 1     | 0     | 1
19 - 25     | 1     | 0     | 1
26 - 45     | 1     | 2     | 3
46 - 60     | 0     | 1     | 1
60 Above    | 0     | 2     | 2

for full syntax here is my sqlfiddle: http://sqlfiddle.com/#!9/e8c12a/2

but the result that I really need is the age with null value also display.
example like this:

Age         | M     | F     | TotalPerson
--------------------------------
0 - 5       | 0     | 0     | 0
6 - 10      | 0     | 0     | 0
11 - 18     | 0     | 0     | 0
19 - 25     | 1     | 0     | 1
19 - 25     | 1     | 0     | 1
26 - 45     | 1     | 2     | 3
46 - 60     | 0     | 1     | 1
60 Above    | 0     | 2     | 2

Answer 1

You may join ages with society:

SELECT 
  Age,
  COALESCE(SUM(gender = 'M'), 0) M,
  COALESCE(SUM(gender = 'F'), 0) F,
  COALESCE(SUM(gender = 'M'), 0) + COALESCE(SUM(gender = 'F'), 0) TotalPerson 
FROM
  (
    SELECT '0 - 5' AS Age, 0 AS age1, 6 AS age2 UNION
    SELECT '6 - 10', 6, 10 UNION
    SELECT '11 - 18', 11, 18 UNION
    SELECT '19 - 25', 19, 25 UNION
    SELECT '26 - 45', 26, 45 UNION
    SELECT '46 - 60', 46, 60 UNION
    SELECT '60 Above', 60, 1000
  ) ages
  LEFT JOIN society ON FLOOR(DATEDIFF(CURDATE(), birdth) / 365) BETWEEN age1 AND age2
GROUP BY Age
ORDER BY age1

Example on SQL Fiddle

Answer 2

There are several things to consider here.

First of all, you should think really hard if you have to do this with SQL. Imho, the query should only responsible for getting the data from the database, and not - in case you do so - that the display of all age groups in a GUI or something like that is complete.

Secondly , the reason that your approach is not working is the way how the query is processed. Simplified in my own words, what MySQL does is this:

1. Get all rows from society table in subquery
2. Execute main query with the result of subquery

Thirdly , and last but not least. If you really HAVE TO do it in MySQL, here would be a doable approach. Create a table with all the age groups you want to have (can also be a temporary table), and then join in the right way (in my example with an RIGHT join, but I think one could also turn the query around and do a LEFT join, you can only not use a INNER JOIN as this negates to possibility to have rows in the final result set that are NULL on one side of the join), so you also get your NULL results. I created a simple example on sqlfiddle based on your approach. Please note however, that it is only a quick and dirty solution (eg I cheated on the max date range by setting to age_to to 1000).

On a side note: I would recommend to not calculate the age by dividing the days by 365. Remember that there are leap years, so over time your result will become more and more inaccurate. Easiest (and most reliable way) for this is imho to subtract the birth year from the current year and check if the date already passed this year, in order to subtract 1 from the result if it did not.