如何计算条件匹配的 GROUP BY 查询的行？

Question

This is my persons table:

neighborhood  birthyear
a             1958
a             1959
b             1970
c             1980

I'd like to get the COUNT of people in an age group within every neighborhood. For example, if I wanted to get everyone under the age of 18, I would get:

neighborhood  count
a             0
b             0
c             0

If I wanted to get everyone over 50, I'd get

neighborhood  count
a             2
b             0
c             0

I tried

SELECT neighborhood, COUNT(*)
    FROM persons
    WHERE YEAR(NOW()) - persons.birthyear < 18
    GROUP BY neighborhood;

but this gives me 0 rows, when instead I want 3 rows with distinct neighborhoods and 0 count for each. How would I accomplish this?

Answer 1

You can use conditional aggregation:

SELECT neighborhood, SUM(YEAR(NOW()) - p.birthyear) as under_18,
       SUM(YEAR(NOW()) - p.birthyear BETWEEN 34 AND 42) as age_34_42
FROM persons p
GROUP BY neighborhood;

Answer 2

I think that if the count is 0, the row doesn't appear. Your code seems correct to me, if you try it on the example with age 50, it should give you one row whith the expected line (neighborhood:a,count:2)

Answer 3

I would recommend using a sub query:

SELECT 
    count(*) [group-by-count-greater-than-ten] 
FROM 
(
   SELECT 
      columnFoo, 
      count(*) cnt
   FROM barTable
   WHERE columnBaz = "barbaz"
   GROUP BY columnFoo
) 
AS subQuery
WHERE cnt > 10

In the above, the subquery return result set is being used by the main query as any other table.

The column cnt is no longer seen by the main query as a computed field and does not have to reference the count() function.

However, inside the subquery running a where clause or a having clause that must look at the alias cnt column, the count() function would have to be referenced as referencing cnt in the subquery would throw an error.

In your case using a subquery would look something like this.

SELECT 
    neighborhood, 
    age, 
    count(*) as cnt
FROM
(
     SELECT 
         *, 
         (YEAR(NOW()) - birthyear) as age 
     FROM PERSONS
) as WithAge
WHERE age < 18
GROUP BY neighborhood, age