Generating unique identifiers as a set based query - sql-server

Question

I am moving a bunch of code over from entirely cursor based to set based and generating this has been doing my head in. We create a 6 character shortcode (unique) for each company inserted into the database and I (want) to achieve this outside of a cursor.
Example of where I am at so far:
CREATE TABLE #customers (name VARCHAR(50), shortname VARCHAR(10))
INSERT INTO #customers VALUES
('Michael Smith', 'Michae')
,('Michael Douglas', 'Mich_1')
,('Michael Yang', 'Mich_2')
CREATE TABLE #newcustomers (name VARCHAR(50), shortname VARCHAR(10) NULL)
INSERT INTO #newcustomers (name) VALUES
('Michael Black')
,('Michael White')
SELECT * FROM #customers
SELECT * FROM #newcustomers
DECLARE #shortname VARCHAR(10)
DECLARE #iteration INT = 0
WHILE EXISTS(SELECT shortname FROM #customers WHERE shortname = #shortname)
BEGIN
SELECT #shortname = LEFT(name, 6) FROM #newcustomers
UPDATE #newcustomers SET shortname = #shortname
SET #shortname = LEFT(#shortname, 4) + '_' + #iteration
SET #iteration = #iteration + 1
END
Hopefully the example is sufficient in identifying where I am trying to get to, any suggestions or examples would be very helpful. My example does not work.

Answer 1

Try this
Your table as mock-up
CREATE TABLE #customers (ID INT IDENTITY, name VARCHAR(50), shortname VARCHAR(10))
INSERT INTO #customers VALUES
('Michael Smith', 'Michae')
,('Michael Douglas', 'Mich_1')
,('Michael Yang', 'Mich_3')
,('Testman', 'Testma')
,('Testman1', 'Test_1');
CREATE TABLE #newcustomers (ID INT IDENTITY,name VARCHAR(50), shortname VARCHAR(10) NULL)
INSERT INTO #newcustomers (name) VALUES
('Michael Black')
,('Michael White')
,('Testman2')
,('Someone new');
--This CTE will combine all existing names
WITH AllNames AS
(
SELECT '1_old' AS datasource,ID,name,shortname FROM #customers
UNION ALL SELECT '2_new',ID,name,shortname FROM #newcustomers
)
--This CTE will use the combined list and calculate the right "index"
,ShortNames AS
(
SELECT c.*
,A.First6
,ROW_NUMBER() OVER(PARTITION BY A.First6 ORDER BY datasource,ID) AS NrTotal
,ROW_NUMBER() OVER(PARTITION BY datasource,A.First6 ORDER BY datasource,ID) AS Nr
,CASE WHEN ISNUMERIC(SUBSTRING(shortname+' ',6,10))=1
THEN CAST(SUBSTRING(shortname+' ',6,10) AS INT) ELSE 0 END AS ExistIndex
FROM AllNames AS c
CROSS APPLY(SELECT LEFT(name + ' ',6)) AS A(First6)
)
--All new with NrTotal=1 get the 6 letters as is, all other get the index
SELECT *
,CASE WHEN datasource='1_old' THEN shortname ELSE
CASE WHEN datasource='2_new' AND NrTotal=1 THEN First6
ELSE LEFT(First6,4) + '_' + CAST(Nr + (SELECT ISNULL(MAX(x.ExistIndex),1)
FROM ShortNames AS x
WHERE x.First6=ShortNames.First6) AS VARCHAR(5))
END
END
FROM ShortNames
GO
DROP TABLE #customers;
DROP TABLE #newcustomers;
The result
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| datasource | ID | name | shortname | First6 | NrTotal | Nr | ExistIndex | (Kein Spaltenname) |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 1_old | 1 | Michael Smith | Michae | Michae | 1 | 1 | 0 | Michae |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 1_old | 2 | Michael Douglas | Mich_1 | Michae | 2 | 2 | 1 | Mich_1 |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 1_old | 3 | Michael Yang | Mich_3 | Michae | 3 | 3 | 3 | Mich_3 |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 1_old | 4 | Testman | Testma | Testma | 1 | 1 | 0 | Testma |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 1_old | 5 | Testman1 | Test_1 | Testma | 2 | 2 | 1 | Test_1 |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 2_new | 1 | Michael Black | NULL | Michae | 4 | 1 | 0 | Mich_4 |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 2_new | 2 | Michael White | NULL | Michae | 5 | 2 | 0 | Mich_5 |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 2_new | 4 | Someone new | NULL | Someon | 1 | 1 | 0 | Someon |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+
| 2_new | 3 | Testman2 | NULL | Testma | 3 | 1 | 0 | Test_2 |
+------------+----+-----------------+-----------+--------+---------+----+------------+--------------------+

Answer 2

One option is to use a computed column.
A table design along these lines would work:
- Sample table.
DECLARE #Sample TABLE
(
Id INT IDENTITY(1, 1),
FullName VARCHAR(255),
ShortName AS LEFT(FullName, 4) + '_' + CAST(Id AS VARCHAR(255))
)
;
-- Add set containing two companies.
INSERT INTO #Sample
(
FullName
)
VALUES
('ABC LTD'),
('XYZ PLC')
;
Returns
Id FullName ShortName
1 ABC LTD ABC _1
2 XYZ PLC XYZ _1
The Id and ShortName columns will be managed by SQL Server. You only need to add the FullName.
EDIT
Reworked example using table variable, to make it easier to play along.