What's the best way to group SQL results by items batch? - sql

Question

For example, I have a simple table books:
author
book
Author-A
Book-A1
Author-A
Book-A2
Author-B
Book-B1
Author-C
Book-C1
Author-C
Book-C2
And I need to count books by each author, so I'll write:
select author, count(*) from books
group by author
# Author-A = 2
# Author-B = 1
# Author-C = 2
But now I need to count books by groups of authors:
groupA = ['Author-A', 'Author-C'],
groupB = ['Author-B']
select authorGroup, count(*) from books
group by {
case author in groupA -> 'groupA'
case author in groupB -> 'groupB'
} as authorGroup
# ['Author-A', 'Author-C'] = 4
# ['Author-B'] = 1
These groups can be different and come from another module.
What's the best way to write this requests? Maybe without union such as:
select author as 'groupA', count(*) from books
where author in { groupA }
union
select author as 'groupB', count(*) from books
where author in { groupB }
because there could be a lot of groups in request (~20-30)
The problem is that these groups can be absolutely dynamic: I can request ['Author-A', 'Author-B'] in one request as one group and ['Author-B', 'Author-C'] in another.
For example, the group is not something like author's country or genre. It can be totally dynamic.

Answer 1

The usual way is to JOIN on to a mapping table, which can be an in-line-view if need be (though I recommend an actual table, which can be indexed).
WITH
author_group AS
(
SELECT 'Author-A' AS author, 'Group-A' AS group_label
UNION ALL
SELECT 'Author-B' AS author, 'Group-B' AS group_label
UNION ALL
SELECT 'Author-C' AS author, 'Group-A' AS group_label
)
SELECT
author_group.group_label,
COUNT(*)
FROM
books
INNER JOIN
author_group
ON author_group.author = books.author
GROUP BY
author_group.group_label
Similar results can be achieved with CASE expressions, but it doesn't scale very well...
WITH
mapped_author AS
(
SELECT
*,
CASE author
WHEN 'Author-A' THEN 'Group-A'
WHEN 'Author-B' THEN 'Group-B'
WHEN 'Author-C' THEN 'Group-A'
END
AS author_group
FROM
books
)
SELECT
author_group,
COUNT(*)
FROM
mapped_author
GROUP BY
author_group

Answer 2

First you need to create a new table that show in what group is the author.
Later you just count
Like this:
select distinct a.group_auth, count(a.book) over (partition by a.group_auth)
from
(select
case when b.Author in [groupA] then 'groupA',
when b.Author in [groupB] then 'groupB'
end case as group_auth,
b.book as book
from books b
) as a
;