Union and order by - sql

Question

Consider a table like
tbl_ranks
--------------------------------
family_id | item_id | view_count
--------------------------------
1 10 101
1 11 112
1 13 109
2 21 101
2 22 112
2 23 109
3 30 101
3 31 112
3 33 109
4 40 101
4 51 112
4 63 109
5 80 101
5 81 112
5 88 109
I need to generate a result set with the top two(2) rows for a subset of family ids (say, 1,2,3 and 4) ordered by view count.
I'd like to do something like
select top 2 * from tbl_ranks where family_id = 1 order by view_count
union all
select top 2 * from tbl_ranks where family_id = 2 order by view_count
union all
select top 2 * from tbl_ranks where family_id = 3 order by view_count
union all
select top 2 * from tbl_ranks where family_id = 4 order by view_count
but, of course, order by isn't valid in a union all context in this manner. Any suggestions? I know I could run a set of 4 queries, store the results into a temp table and select the contents of that temp as the final result, but I'd rather avoid using a temp table if possible.
Note: in the real app, the number of records per family id is indeterminate, and the view_counts are also not fixed as they appear in the above example.

Answer 1

You can try something like this
DECLARE #tbl_ranks TABLE(
family_id INT,
item_id INT,
view_count INT
)
INSERT INTO #tbl_ranks SELECT 1,10,101
INSERT INTO #tbl_ranks SELECT 1,11,112
INSERT INTO #tbl_ranks SELECT 1,13,109
INSERT INTO #tbl_ranks SELECT 2,21,101
INSERT INTO #tbl_ranks SELECT 2,22,112
INSERT INTO #tbl_ranks SELECT 2,23,109
INSERT INTO #tbl_ranks SELECT 3,30,101
INSERT INTO #tbl_ranks SELECT 3,31,112
INSERT INTO #tbl_ranks SELECT 3,33,109
INSERT INTO #tbl_ranks SELECT 4,40,101
INSERT INTO #tbl_ranks SELECT 4,51,112
INSERT INTO #tbl_ranks SELECT 4,63,109
INSERT INTO #tbl_ranks SELECT 5,80,101
INSERT INTO #tbl_ranks SELECT 5,81,112
INSERT INTO #tbl_ranks SELECT 5,88,109
SELECT *
FROm (
SELECT *,
ROW_NUMBER() OVER(PARTITION BY family_id ORDER BY view_count DESC) MyOrder
FROM #tbl_ranks
) MyOrders
WHERE MyOrder <= 2

Answer 2

If you're using SQL Server 2005 or later, you can take advantage of analytic functions:
SELECT * FROM (
SELECT rank() OVER (PARTITION BY family_id ORDER BY view_count) AS RNK, * FROM ...
)
WHERE RNK <= 2
ORDER BY ...

Answer 3

SELECT tro.*
FROM family
CROSS APPLY
(
SELECT TOP 2 *
FROM tbl_ranks tr
WHERE tr.family_id = family.id
ORDER BY
view_count DESC
) tro
WHERE family.id IN (1, 2, 3, 4)
If you don't have an actual family table, you can construct it using a set of unions or a recursive CTE:
WITH family AS
(
SELECT 1 AS id
UNION ALL
SELECT 2 AS id
UNION ALL
SELECT 3 AS id
UNION ALL
SELECT 4 AS id
)
SELECT tro.*
FROM family
CROSS APPLY
(
SELECT TOP 2 *
FROM tbl_ranks tr
WHERE tr.family_id = family.id
ORDER BY
view_count DESC
) tro
WHERE family.id IN (1, 2, 3, 4)
Make sure you have an index on tbl_ranks (family_id, viewcount).
This will be efficient if you have lots of ranks per family, since analytic functions like ROW_NUMBER will not use the TOP method if used with PARTITION BY.

Answer 4

Use:
SELECT *
FROM (select *,
ROW_NUMBER() OVER (PARTITION BY family_id ORDER BY view_count DESC) 'rank'
from tbl_ranks) x
WHERE x.rank <= 2
ORDER BY ...
The rationale is to assign a ranking, and then filter based on it.

Answer 5

You only have to tweak your suggested SQL commands a little bit to make it work like you wanted. To bind TOP and ORDER BY you can put the statement inside paranthesis which you select from and give a name (not used here but required).
With the DECLARE and INSERT statements from Adriaan Stander's answer the following
SELECT * FROM (SELECT TOP 2 * FROM #tbl_ranks WHERE family_id = 1 ORDER BY view_count) AS dummy1 UNION ALL
SELECT * FROM (SELECT TOP 2 * FROM #tbl_ranks WHERE family_id = 2 ORDER BY view_count) AS dummy2 UNION ALL
SELECT * FROM (SELECT TOP 2 * FROM #tbl_ranks WHERE family_id = 3 ORDER BY view_count) AS dummy3 UNION ALL
SELECT * FROM (SELECT TOP 2 * FROM #tbl_ranks WHERE family_id = 4 ORDER BY view_count) AS dummy4
gives
family_id item_id view_count
1 10 101
1 13 109
2 21 101
2 23 109
3 30 101
3 33 109
4 40 101
4 63 109