Is there a good way to do this in SQL? - sql

Question

I am trying to solve the following problem entirely in SQL (ANSI or TSQL, in Sybase ASE 12), without relying on cursors or loop-based row-by-row processing.
NOTE: I already created a solution that accomplishes the same goal in application layer (therefore please refrain from "answering" with "don't do this in SQL"), but as a matter of principle (and hopefully improved performance) I would like to know if there is an efficient (e.g. no cursors) pure SQL solution.
Setup:
I have a table T with the following 3 columns (all NOT NULL):
---- Table T -----------------------------
| item | tag | value |
| [int] | [varchar(10)] | [varchar(255)] |
The table has unique index on item, tag
Every tag has a form of a string "TAG##" where "##" is a number 1-99
Existing tags are not guaranteed to be contiguous, e.g. item 13 may have tags "TAG1", "TAG3", "TAG10".
TASK: I need to insert a bunch of new rows into the table from another table T_NEW, which only have items and values, and assign new tag to them so they don't violate unique index on item, tag.
Uniqueness of values is irrelevant (assume that item+value is always unique already).
---- Table T_NEW --------------------------
| item | tag | value |
| [int] | STARTS AS NULL | [varchar(255)] |
QUESTION: How can I assign new tags to all rows in table T_NEW, such that:
All item+tag combinations in a union of T and T_NEW are unique
Newly assigned tags should all be in the form "TAG##"
Newly assigned tags should ideally be the smallest available for a given item.
If it helps, you can assume that I already have a temp table #tags, with a "tag" column that contains 99 rows containing all the valid tags (TAG1..TAG99, one per row)

Answer 1

I started a fiddle that will get you the list of available "open" tags by item. It does this using the #tags (AllTags) and doing an outer-join-where-null. You could use that to insert new tags from T_New...
with T_openTags as (
select
items.item,
openTagName = a.tag
from
(select distinct item from T) items
cross join AllTags a
left outer join T on
items.item = T.item
and T.tag = a.tag
where
T.item is null
)
select * from T_openTags
or see this updated fiddle to do an update on T_New table. Essentially adds a row_number so we can pick the correct open tag to use in a single update statement. I padded the Tag names with a leading zero to simplify the sorting.
with T_openTags as (
select
items.item,
openTagName = a.tag,
rn = row_number() over(partition by items.item order by a.tag)
from
(select distinct item from T) items
cross join AllTags a
left outer join T on
items.item = T.item
and T.tag = a.tag
where
T.item is null
), T_New_numbered as (
select *,
rn = row_number() over(partition by item order by value)
from T_New
)
update tnn set tag = openTagName
from T_New_numbered tnn
inner join T_openTags tot on
tot.item = tnn.item
and tot.rn = tnn.rn
select * from T_New
updated fiddle with poor mans row_number replacement that only works with distinct T_New values

Answer 2

Try this:
DECLARE #T TABLE (ITEM INT, TAG VARCHAR(10), VALUE VARCHAR(255))
INSERT INTO #T VALUES
(1,'TAG1', '100'),
(2,'TAG2', '200')
DECLARE #T_NEW TABLE (ITEM INT, TAG VARCHAR(10), VALUE VARCHAR(255))
INSERT INTO #T_NEW VALUES
(3,NULL, '500'),
(4,NULL, '600')
INSERT INTO #T
SELECT
ITEM,
('TAG' + CONVERT(VARCHAR(20),ITEM)) AS TAG,
VALUE
FROM
#T_NEW
SELECT * FROM #T

Answer 3

OK, here's a correct solution, tested to work on Sybase (H/T: big thanks to #ypercube for providing a solid basis for it)
declare #c int
select #c = 1
WHILE (#c > 0)
BEGIN
UPDATE
t_new
SET
tag =
( SELECT min(tags.tag)
FROM #tags tags
LEFT JOIN t o
ON tags.tag = o.tag
AND o.item = t_new.item
LEFT JOIN t_new n3
ON tags.tag = n3.tag
AND n3.item = t_new.item
WHERE o.tag IS NULL
AND n3.tag IS NULL
)
WHERE tag IS NULL
-- and here's the main magic for only updating one item at a time
AND NOT EXISTS (SELECT 1 FROM t_new n2 WHERE t_new.value > n2.value
and n2.tag IS NULL and n2.item=t_new.item)
SELECT #c = ##rowcount
END

Answer 4

Inserting directly to t:
INSERT INTO t
(item, tag, value)
SELECT
item,
( SELECT MIN(tags.tag)
FROM #tags AS tags
LEFT JOIN t AS o
ON tags.tag = o.tag
AND o.item_id = n.item_id
WHERE o.tag IS NULL
) AS tag,
value
FROM
t_new AS n ;
Updating t_new:
UPDATE
t_new AS n
SET
tag =
( SELECT MIN(tags.tag)
FROM #tags AS tags
LEFT JOIN t AS o
ON tags.tag = o.tag
AND o.item_id = n.item_id
WHERE o.tag IS NULL
) ;
Correction
UPDATE
n
SET
n.tag = w.tag
FROM
( SELECT item_id,
tag,
ROW_NUMBER() OVER (PARTITION BY item_id ORDER BY value) AS rn
FROM t_new
) AS n
JOIN
( SELECT di.item_id,
tags.tag,
ROW_NUMBER() OVER (PARTITION BY di.item_id ORDER BY tags.tag) AS rn
FROM
( SELECT DISTINCT item_id
FROM t_new
) AS di
CROSS JOIN
#tags AS tags
LEFT JOIN
t AS o
ON tags.tag = o.tag
AND o.item_id = di.item_id
WHERE o.tag IS NULL
) AS w
ON w.item_id = n.item_id
AND w.rn = n.rn ;