Re: Extend inner join to fetch not yet connected rows also

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От John W Higgins
Тема Re: Extend inner join to fetch not yet connected rows also
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Msg-id CAPhAwGyGGMYKB1Xd3VKCysDuWpw3Ufv277qZB2+4bVRvWbKz8w@mail.gmail.com
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Ответ на Re: Extend inner join to fetch not yet connected rows also  (Arup Rakshit <ar@zeit.io>)
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On Sun, Sep 22, 2019 at 6:30 AM Arup Rakshit <ar@zeit.io> wrote:


SELECT
        craftsmanships.id,
        craftsmanships.name,
        CASE WHEN contractor_skills.user_id IS NULL THEN
                FALSE
        ELSE
                TRUE
        END AS has
FROM
        "craftsmanships"
        LEFT JOIN "contractor_skills" ON "contractor_skills"."craftsmanship_id" = "craftsmanships"."id"
        LEFT JOIN "users" ON "users"."id" = "contractor_skills"."user_id"
WHERE (contractor_skills.user_id = 8
        OR contractor_skills.user_id IS NULL)
ORDER BY
        "craftsmanships"."id”;

Gives correct result. Not sure if still this query has bug in it.


If you do not understand the query - then it's wrong on its face. You should never run something which you do not understand.

So one should take a step back - make smaller pieces and then combine smaller pieces of logic together to form an answer. If at some point in the future there is a performance issue - then deal with that then - but do not make some fancy multi join query that you do not fully understand.

So in that vein,

Piece 1 = A list of craftsmanship_id for a particular user
Piece 2 - Take piece 1 and compare to the full list of craftsmanship_id

Putting piece 1 into a CTE you end up with something like this.

with UserSkills as (
SELECT 
  craftsmanship_id
FROM
  contractor_skills
WHERE
  user_id = 3
)
SELECT
  CASE WHEN UserSkills.ctraftsmanship_id IS NULL THEN FALSE
  ELSE TRUE as has
FROM
  craftsmanships
LEFT JOIN
  UserSkills
ON
  craftsmanships.id = UserSkills.craftsmanship_id

So you take the two pieces and combine then. Yes you can drop the CTE into the main body - but unless you are certain you are doing it correctly - there is no point doing that. The query parser will do the work for you - so why bother making your life more difficult then it need be.

John W Higgins
 

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