How to modify duplicate strings in SQL Server? - sql

Question

I'm dealing with truncating a column of string values in a table from 15 characters to 10 characters (this is the new max length I want to permit for the column).
There is a unique key on a pair of columns in the table, this one being one of them.
Because of the truncation, there is a possibility that this could be violated.
For example:
| ID | C1 | C2 |
| -- | --------------- | -- |
| 1 | 123456789012345 | 1 |
| 2 | 123456789012346 | 1 |
| 3 | 123456789012345 | 2 |
| 4 | 123456789012346 | 2 |
Let's say I have a unique key on C1 and C2. C1 is currently varchar(15), but for reasons that are beyond my control, it's being changed to varchar(10).
I have to truncate the values in C1 to strings of length 10. But if I just do so mindlessly, I'll obviously end up (in the example above) violating the unique key constraint.
So, I know how to find all the duplicates using something like:
select
t1.ID,
LEFT(t1.C1, 10) as C1,
t1.C2
INTO
#ColumnDuplicates
FROM
t t1
join t t2 on
t1.ID <> t2.ID
AND LEFT(t1.C1, 10) = LEFT(t2.C1, 10)
WHERE
t1.C2 = t2.C2
SELECT * FROM #ColumnDuplicates
Referring to the table above, this query would get me:
| ID | C1 | C2 |
| -- | ---------- | -- |
| 1 | 1234567890 | 1 |
| 2 | 1234567890 | 1 |
| 3 | 1234567890 | 2 |
| 4 | 1234567890 | 2 |
Now here's where I'm not sure how to do the next step. What I need to do is somehow get to this:
| ID | C1 | C2 |
| -- | ---------- | -- |
| 1 | 123456_001 | 1 |
| 2 | 123456_002 | 1 |
| 3 | 123456_001 | 2 |
| 4 | 123456_002 | 2 |
Effectively, I want to find all the duplicate C1 values for each C2 value, and then change the last 4 characters to a _[0-9][0-9][0-9] pattern, and progressively number those duplicates from 000 (or 001, I don't really care which is used as the starting point) through to a maximum of 999. This will give me space to deal with around 999 duplicates per C2 value, which I am quite sure based on my familiarity with the data I'm working with will not be an issue.
And then I can easily just use this temporary table to update the C1 values in the main table I am modifying.
My knowledge of SQL at the moment is quite basic, so I don't really know how to accomplish this.

Answer 1

If you are lucky, you can look at duplicates in the first six characters. I say lucky, because this assumes you never have more than 1000 such duplicates:
with toupdate as (
select t.*,
row_number() over (partition by left(c1, 6), c2 order by c2) as seqnum,
count(*) over (partition by left(c1, 6), c2) as cnt
from t
)
update toupdate
set c1 = (case when cnt > 1
then concat(left(c1, 6), '_', format(seqnum, '000'))
else left(c1, 10)
end);
The above is a little pessimistic with respect to duplicates. It probably makes sense to filter out known singletons before using row_number():
with toupdate as (
select t.*,
row_number() over (partition by left(c1, 6), c2,
(case when cnt10 > 1 then 1 else 2 end)
order by c2
) as seqnum,
count(*) over (partition by left(c1, 6), c2,
(case when cnt10 > 1 then 1 else 2 end)
) as cnt6
from (select t.*,
count(*) over (partition by left(c1, 10), c2) as cnt10
from t
) t
)
update toupdate
set c1 = (case when cnt10 > 1
then concat(left(c1, 6), '_', format(seqnum, '000'))
else left(c1, 10)
end);

Answer 2

You can use an updatable CTE to achieve this:
CREATE TABLE dbo.YourTable (ID int NOT NULL,
C1 varchar(15) NOT NULL,
C2 int NOT NULL);
CREATE UNIQUE INDEX YourIndex ON dbo.YourTable (C1,C2);
GO
INSERT INTO dbo.YourTable (ID, C1, C2)
VALUES (1,'123456789012345',1),
(2,'123456789012346',1),
(3,'123456789012345',2),
(4,'123456789012346',2);
GO
WITH CTE AS(
SELECT C1,
LEFT(YT.C1,6) + '_' + RIGHT(CONCAT('000',ROW_NUMBER() OVER (ORDER BY YT.C1, YT.C2 ASC)),3) AS NewC1
FROM dbo.YourTable YT
WHERE LEN(YT.C1) > 10) --Unsure if that WHERE is needed
UPDATE CTE
SET C1 = NewC1;
GO
DROP INDEX YourIndex ON dbo.YourTable; --Has to be dropped to alter
ALTER TABLE dbo.YourTable ALTER COLUMN C1 varchar(10) NOT NULL;
GO
CREATE UNIQUE INDEX YourIndex ON dbo.YourTable (C1,C2); --Recreate
GO
SELECT *
FROM dbo.YourTable;
GO
DROP TABLE dbo.YourTable;