雪花中的子查询与未知匹配，与列匹配

Question

I have a few tables that I'm joining and one of them can have an unknown number of matches, up to 6. Each match should be returned as a row value in the initial query. For example:

SELECT a.ID, a.match1, a.match2, a.match3, a.match4, a.match5, a.match6
FROM table1 a, (SELECT ID, match FROM table2 WHERE a.ID = table2.ID) b
WHERE a.ID = b.ID

That's probably not the right syntax but hopefully it shows what I need. So the nested query MAY return 1 match or 5. Each match should be the value for the corresponding column name, ie a.match1 = first match, b.match2 = second match, etc etc.

Please let me know if I need to explain further. I know this isn't the optimal schema to use but it's what I was told to work with.

Answer 1

Something that SQL doesn't like is an unknown number of columns.

As a quick hack, you could aggregate all matches in an array, and then have a query around it transforming the matches into a predefined (large) number of columns.

Like this:

with data as (
select $1 id
from (values(1),(2))
), data2 as (
select $1 id, $2 match
from (values(1, 'a1'),(1, 'a2'),(2, 'b1'),(2, 'b2'),(2, 'b3'))
)

select id, matches[0], matches[1], matches[2], matches[3]
from (
    select a.id, array_agg(match) matches
    from data a
    join data2 b
    on a.id=b.id
    group by 1
);