Question

My table content:

id  aid
----------
1   1
2   2
3   2
4   4
5   4

my sql: select count(*) from table where aid in (1,2,3,4) group by aid

result:

count(*)
----------
1
2
2

I would like result:

count(*)
----------
1
2
0
2

I want a count result when aid is 1,2,3,4. What a sql should I use? Thanks for help.
（I‘m use MySQL）

Answer 1

Time for an obvious statement:
– Data can’t “magically appear”, it needs to exist somewhere for it to appear in the output.

This is relevant for your desire to have a count of 0 for aid = 3. Because there isn’t a record with aid = 3, it can’t be counted. It’s data that doesn’t exist so will never appear in the output. So we need to force it to exist.

If you have a table that lists all of the aid entities, that’s easy to do with a LEFT JOIN…

SELECT
  aid.id,
  COUNT(yourDataTable.aid) AS data_count
FROM
  aid
LEFT JOIN
  yourDataTable
    ON aid.id = yourDataTable.aid
WHERE
  aid.id IN (1,2,3,4)
GROUP BY
  aid.id

(COUNT(yourDataTable.aid) does not count records where the field is NULL. So the 3 now exists, and the COUNT() can return 0.)