sql to group table of product owners with a column for primary owner and concatenated secondary owners

问题

I have a table of products with their owners. Each owner is in their own row and has an owner type of either primary or secondary. Not every product has a secondary owner.

I need to get a table grouped by product with the primary owner in one column and all the secondary owners concatenated in a second column. If a product has multiple primary owners it should select the first one and the rest go to the secondary owners. If a product doesn't have a primary owner then it should just select the first/any secondary owner.

This is an input table:

+---------+------------+----------+
| Product | Owner Type |  Owner   |
+---------+------------+----------+
| a       | primary    | one      |
| a       | secondary  | two      |
| a       | secondary  | three    |
| b       | primary    | four     |
| b       | secondary  | five     |
| c       | primary    | six      |
| d       | secondary  | seven    |
| e       | secondary  | eight    |
| e       | secondary  | nine     |
| f       | primary    | ten      |
| f       | primary    | eleven   |
| f       | secondary  | twelve   |
| f       | secondary  | thirteen |
+---------+------------+----------+

The expected result is:

+---------+---------------+--------------------------+
| Product | Primary Owner |     Secondary Owners     |
+---------+---------------+--------------------------+
| a       | one           | two, three               |
| b       | four          | five                     |
| c       | six           |                          |
| d       | seven         |                          |
| e       | eight         | nine                     |
| f       | ten           | eleven, twelve, thirteen |
+---------+---------------+--------------------------+

If you notice, products d and e don't have a primary owner so it picks the first secondary owner and then doesn't include it again in the secondary owner column. Similar for product f that has two primary owner.

I know how to group by product and use FOR XML PATH to concatenate rows/fields. In the group I know how to select the first product where Owner Type is primary. What I cannot figure out is the logic that would be needed to pick the first primary owner and exclude it from the secondary owner column and/or select the first secondary owner if there is no primary owner and exclude it from the secondary owner column.

I don't even know where to start with the SQL.

Any ideas?

回答1:

One way to do this is assigning row numbers prioritizing owner_type='Primary' rows. Then get the first row as the primary owner and group_concat others to be secondary owners.

select product
,max(case when rnum=1 then owner end) as primary_owner
,group_concat(case when rnum<>1 then owner end order by rnum) as secondary_owners
from (select product,owner_type,owner,
      @rn:=case when @prev_product=product then @rn+1 else 1 end as rnum,
      @prev_product:=product 
      from tablename 
      cross join (select @rn:=0,@prev_product:='',@prev) r
      order by product,owner_type='Primary',owner
     ) t
group by product
order by 1

Sample Demo

来源：https://stackoverflow.com/questions/43746314/sql-to-group-table-of-product-owners-with-a-column-for-primary-owner-and-concate