mysql - 从没有所有组合的多个表中获取相关行

Question

Let's say I have a primary table record , and it has 2 related tables fields and comments .

CREATE TABLE record (id int primary key, name varchar(20))
CREATE TABLE fields (record_id int, name varchar(20), val int)
CREATE TABLE comments (record_id int, who varchar(20), comment text)

I would like to run one query that fetches a set of records AND fetches all the related fields for that record, AND fetches all the comments related to that record.

If I do left joins to ensure I get the records, I would use:

select * from record
     left join fields on (fields.record_id = record.id)
     left join comments on (comments.record_id = record.id)
     order by record.id

The problem is, I get back n * m rows for each record, where n is the number of fields, and m is the number of comments. I want to get back n + m rows (what makes sense is that the fields columns are all null while returning the comments, and the comments columns are all null while returning the fields). Is there a way to make this work aside from inserting a dummy comment and dummy field to join with? I would very much prefer not to have to perform an extra query for each record.

I suppose this is not mysql specific, but that's what I'm using for my application.

Answer 1

I get back n * m rows for each record, where n is the number of fields, and m is the number of comments. I want to get back n + m rows

    SELECT * 
    FROM record
    LEFT JOIN fields ON (fields.record_id = record.id) /* maybe INNER JOIN ? */
    LEFT JOIN comments ON (1=0)
UNION ALL
    SELECT * 
    FROM record
    LEFT JOIN fields ON (1=0)
    LEFT JOIN comments ON (comments.record_id = record.id) /* maybe INNER JOIN ? */
-- ORDER BY record.id

Answer 2

I want to get back n + m rows (what makes sense is that the fields columns are all null while returning the comments, and the comments columns are all null while returning the fields)

One option uses union all in a subquery, then a left join :

select *
from record r
left join (
    select record_id, name, val, null who, null comment from fields
    union all record_id, select null, null, who, comment from comments
) x on x.record_id = r.record_id

That's a weird resultset though.