Unable to make a suitable query

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I'm trying to make a database. It has following subset of tables

  1. jobs(job_id, description)
  2. skills (skill_id, skills)
  3. jobs_and_skills (job_id, skill_id)
  4. applicant_skill (applicant_id , skills_id)

My query must be : Find applicants who have skills required by a given jobs id

I did something like this using a series of subqueries

select DISTINCT applicant_id
from applicant_skill
where  skill_id in
(select DISTINCT skill_id
from jobs_and_skill
where  job_id in (select job_id
from jobs
where description ="research"));

But here if I have a job that needs skills say A,B,C and I have candidate which has skill A only it will return that candidate also. Is there a way to find candidate that has all skills A,B,C and not return candidates with just a subset of required skills

Use JOIN rather than IN. Then you can join with a subquery that counts the number of skills a job requires, and compare that with the count of skills the applicant has in common.

SELECT as.applicant_id
FROM applicant_skill AS as
JOIN jobs_and_skills AS js ON js.skill_id = as.skill_id
JOIN jobs AS j ON j.job_id = js.job_id
JOIN (
    SELECT job_id, COUNT(*) AS skill_count
    GROUP BY job_id
) AS jsc ON j.job_id = jsc.job_id
WHERE j.description = "research"
GROUP BY as.applicant_id
HAVING COUNT(*) = jsc.skill_count

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You can prepare subquery for jobs_and_skill before joining to applicants.

select * from applicant_skill t1
inner join 
    (select t2.skill_id from 
        jobs_and_skill t2
        inner join skills t3 on t3.skill_id = t2.skill_id
        inner join jobs t4 on t4.job_id = t2.job_id
        where t4.description  = 'research') as t 
    on t1.skill_id = t.skill_id

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You could join jobs and applicants with a condition that ensures that the applicant has all of job skills.

select 
    j.*,
    a.*
from 
    jobs j
    inner join applicants a on not exists (
        select 1
        from job_and_skills js
        left join applicant_skills as
            on  as.applicant_id = a.applicant_id
            and as.skill_id = js.skill_id
        where 
            js.job_id = j.job_id
            and as.applicant_id is null
    )
where j.description = 'research'

The join condition uses a not exists condition uses a left join anti-pattern that phrases as follows: there is no required skill for this job that the applicant does not own.

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Comments
  • Shouldn't the second column in applicant_skill be skill_id instead of skills?
  • Also, don't you have a separate table for applicants, like (applicant_id, name, ...)?
  • @GMB indeed I have a separate table for applicantsand also applicant_skill should have skill_id as second column