SQL Update question - sql

Question

I am wondering how this can be achieved.
Let's say I have a table with two columns (IU(uniqueidentifier),(ID(int), SEL(char(1))
ID column has the following values in each row(ordered by IU):
0, 1, 2, 2, 0, 0, 1, 2, 2, 2, 0, 0, 4, 2, 2, 0, 0, 1, 2, 0, 0
I need to update column SEL with 'Y' for rows which are part of the group:
1, 2, 2, 2 ...
(Starts With 1 and in the next rows thare are 2's. (Group 4, 2, 2 is not correct).
So in this example column: SEL should be:
null, Y, Y, Y, null, null, Y, Y, Y, Y, null, null, 4, 2, 2, null, null, Y, Y, null, null
Thanks!

Answer 1

Here's a set-based approach.
DDL & sample data:
DECLARE #atable TABLE (
UI uniqueidentifier DEFAULT NEWSEQUENTIALID(),
ID int,
SEL char(1)
);
INSERT INTO #atable (ID)
SELECT 0 UNION ALL
SELECT 1 UNION ALL
SELECT 2 UNION ALL
SELECT 2 UNION ALL
SELECT 0 UNION ALL
SELECT 0 UNION ALL
SELECT 1 UNION ALL
SELECT 2 UNION ALL
SELECT 2 UNION ALL
SELECT 2 UNION ALL
SELECT 0 UNION ALL
SELECT 0 UNION ALL
SELECT 4 UNION ALL
SELECT 2 UNION ALL
SELECT 2 UNION ALL
SELECT 0 UNION ALL
SELECT 0 UNION ALL
SELECT 1 UNION ALL
SELECT 2 UNION ALL
SELECT 0 UNION ALL
SELECT 0;
The UPDATE statement:
WITH marked AS (
SELECT
*,
grp = CASE ID WHEN 0 THEN 0 ELSE 1 END
FROM #atable
),
grouped AS (
SELECT
*,
grpID = ROW_NUMBER() OVER (ORDER BY UI)
- ROW_NUMBER() OVER (PARTITION BY grp ORDER BY UI)
FROM marked
),
ranked AS (
SELECT
*,
rnk = ROW_NUMBER() OVER (PARTITION BY grp, grpID ORDER BY UI)
FROM grouped
)
UPDATE g
SET SEL = CASE r.ID
WHEN 0 THEN NULL
WHEN 1 THEN 'Y'
ELSE CAST(g.ID AS varchar)
END
FROM grouped g
INNER JOIN ranked r ON g.grp = r.grp AND g.grpID = r.grpID
WHERE r.rnk = 1;
The result of SELECT * FROM #atable after the update:
UI ID SEL
------------------------------------ ----------- ----
A4095E70-A0CC-E011-813B-20CF30905E89 0 NULL
A5095E70-A0CC-E011-813B-20CF30905E89 1 Y
A6095E70-A0CC-E011-813B-20CF30905E89 2 Y
A7095E70-A0CC-E011-813B-20CF30905E89 2 Y
A8095E70-A0CC-E011-813B-20CF30905E89 0 NULL
A9095E70-A0CC-E011-813B-20CF30905E89 0 NULL
AA095E70-A0CC-E011-813B-20CF30905E89 1 Y
AB095E70-A0CC-E011-813B-20CF30905E89 2 Y
AC095E70-A0CC-E011-813B-20CF30905E89 2 Y
AD095E70-A0CC-E011-813B-20CF30905E89 2 Y
AE095E70-A0CC-E011-813B-20CF30905E89 0 NULL
AF095E70-A0CC-E011-813B-20CF30905E89 0 NULL
B0095E70-A0CC-E011-813B-20CF30905E89 4 4
B1095E70-A0CC-E011-813B-20CF30905E89 2 2
B2095E70-A0CC-E011-813B-20CF30905E89 2 2
B3095E70-A0CC-E011-813B-20CF30905E89 0 NULL
B4095E70-A0CC-E011-813B-20CF30905E89 0 NULL
B5095E70-A0CC-E011-813B-20CF30905E89 1 Y
B6095E70-A0CC-E011-813B-20CF30905E89 2 Y
B7095E70-A0CC-E011-813B-20CF30905E89 0 NULL
B8095E70-A0CC-E011-813B-20CF30905E89 0 NULL

Answer 2

Rows in a table have no inherent order, so your grouping (1,2,2,2) is completely arbitrary. It is not guaranteed that your id's will always come in this order:
0, 1, 2, 2, 0, 0, 1, 2, 2, 2, 0, 0, 4, 2, 2, 0, 0, 1, 2, 0, 0
It could be that they come in a completely other order. So you need to specify a ORDER BY clause to get your order. As you have no other fields in your table but SEL and ID, I suppose this is not possible.

Answer 3

I really hope someone comes up with something better than this, because I hate this answer.
create table #test (
IU int identity primary key,
id int,
sel varchar(1)
)
insert into #test(id)
values (0), (1), (2), (2), (0), (0), (1), (2), (2), (2), (0), (0), (4), (2), (2), (0), (0), (1), (2), (0), (0)
DECLARE myCur CURSOR FORWARD_ONLY
FOR
select t.ID
from #test t
order by t.IU
FOR UPDATE OF t.sel
DECLARE #ID int, #lagSel varchar(1)
OPEN myCur
FETCH myCur INTO #ID
WHILE (##FETCH_STATUS = 0) BEGIN
SET #lagSel = CASE
WHEN #lagSel = 'Y' AND #ID in (1,2) THEN 'Y'
WHEN #ID = 1 THEN 'Y'
ELSE NULL
END
UPDATE #test
SET sel = #lagSel
WHERE CURRENT OF myCur
FETCH myCur INTO #ID
END
CLOSE myCur
DEALLOCATE myCur
A couple of things to note:
We're manually managing the value of #lagSel within the cursor so we can carry a value from one row to the next.
In order to be able to use the cursor FOR UPDATE, the table has to have a primary key.
In the UPDATE statement, the WHERE CURRENT OF myCur gives (at least in theory) a big performance gain over any other where clause.
I first tried doing this with lagged joins, but couldn't quite get it there. Here's my work in case someone else can do better:
select main.IU, main.id,
CASE
WHEN main.id = 1 THEN 'Y'
WHEN main.id = 2 AND lag.id in (1, 2) THEN 'Y'
ELSE NULL
END as new_sel
from #test main left outer join
#test lag on main.IU = lag.IU + 1

Answer 4

I think you have the design error here. MS SQL Server does not knows nothing about "next" and "previous" rows. if you try to select the records, the order of the records can be changed from time to time, unless you specify the ordering using ORDER BY statement.
I think you need to change the structure of the tables first.
EDIT: As I see, you have the field and can order your records. Now you can achive your goal using CURSOR.
Briefly, you can create the CURSOR FOR SELECT IU, ID ORDER BY IU ASC.
looping through the cursor records you can check the sequence of the values of ID field, and when sequence will be fully equivalent, you can update the corresponding record.