MySQL从子查询中分离出组子句

Question

Here is my query

SELECT 
    COALESCE(js.name,'Lead saknas'),
    count(j.id)
FROM 
    jobs j
LEFT JOIN
    job_sources js
    ON j.job_source=js.id
LEFT JOIN 
    (SELECT 
        * 
    FROM 
        quotes 
    GROUP BY 
        job_id) q 
    ON j.id=q.job_id
GROUP BY
    j.job_source

The problem is that it's allowed for each job to have more than one quote. Because of that i group the quotes by job_id. Now sure, this works. But i don't like the solution with a subquery. How can i break out the group clause from the subquery to the main query? I have tried to add q.job_id to the main group clause, both before and after the existing one but don't get the same results.

Answer 1

The GROUP BY in the subquery limits all results from quotes to 1 for a given job_id (MySQL will only return 1 result for each distinct grouping if you just do 'SELECT *', even if there are multiple rows that has that grouping), so you're joining jobs onto job_sources onto a table with only 1 row from quotes for each job_id.

What would happen when the GROUP BY is in the main query, is that it will join the 3 tables, then select 1 of them for each job_id.

Since MySQL can return any 1 of multiple rows, obviously you cannot expect the results to be the same in every case.

If you were to use group functions it should return the same results (try something like SELECT COUNT(*), GROUP_CONCAT(quote_id) FROM quotes).

MS SQL doesn't allow you just to do a 'SELECT *' when doing a GROUP BY (unless you GROUP BY all the fields, which is kind of pointless), since it can return different results for different queries. Being allowed to do this in MySQL means you should know what you're doing.

More specifically MS SQL does not allow you to select fields which you did not group on, unless you use grouping functions on them.

Answer 2

You are looking for a simple group by , I think:

SELECT COALESCE(js.name,'Lead saknas'), count(j.id)
FROM jobs j LEFT JOIN
     job_sources js
     ON j.job_source=js.id
GROUP BY COALESCE(js.name,'Lead saknas')