MySQL JOIN需要附加条件

Question

I have two tables, a jobs table and an applicants table. The applicants table has a field containing the position applied to. My query seems close, except it's returning only jobs that have been applied to. What I want is a display of all active jobs that also has a column to display the number of applicants. Some jobs have no applicants and those are not captured by my query.

SELECT * , COUNT(*) as total
FROM `job` 
INNER JOIN applicants 
ON applicants.position_id=job.id
WHERE active = 'yes'
GROUP BY position_id
ORDER BY title

Answer 1

You want a left join and other small fixes to the query:

SELECT j.*, COUNT(a.position_id) as total
FROM `job` j LEFT JOIN
     applicants a
     ON a.position_id = j.id
WHERE j.active = 'yes'
GROUP BY j.id 
ORDER BY j.title;

The changes are:

• Introduced table aliases (makes the query easier to write and to read).
• Use the table aliases for all column references.
• Changed select to j.* rather than * . Only the job information is appropriate because of the group by .
• Changed group by to j.id , so it never aggregates by NULL .
• Changed the count() to count matches.

Answer 2

As other have mentioned, your join would have to be an outer join.

With your simple query, however, you can easily select from job only and get the application count in the select clause. So you get the query even more readable.

select 
  job.*,
  (select count(*) from applicants where position_id = job.id) as total
from job
where active = 'yes'
order by title;