MSSQL Can a column auto-increment only under certain circumstances - sql-server

Question

I have a table with 4 columns ID, c1, c2 and LOT. ID is the primary key. For every record when c1 is 5 I want to auto-generate a number for LOT which will be a sequence starting from 1 for each distinct value of c2.
So if c1 is not 5, LOT remains null. But if c1 is 5 then for every record where c2=1 I want to populate LOT with an auto-incrementing sequence starting from 1.
Ex:
ID c1 c2 LOT
1 3
2 5 1 1
3 5 1 2
4 5 1 3
5 4
Then do the same for a different value of c2. So if c2 is 2, have another bunch of auto-incrementing LOT numbers starting from 1:
ID c1 c2 LOT
6 3
7 5 2 1
8 5 1 4
9 5 2 2
10 5 2 3
We are using MSSQL 2014 Enterprise Ed. Would table-partitioning be useful or do I need to create special tables for each distinct value of C2?

Answer 1

not with an identity field, you can use a trigger instead.

Answer 2

There is no way of doing this using the Identity feature, however, consider using a Instead of trigger to manually manage the values like you want.

Answer 3

You could use the logic to generate LOT in a query or view:
SELECT ID, C1, C2,
CASE
WHEN C1<>5 OR C2 IS NULL THEN NULL
ELSE COUNT(*) OVER (PARTITION BY C1, C2 ORDER BY ID ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
END AS LOT
FROM D
ORDER BY ID
Given a table generated with (ID, C1, C2):
CREATE TABLE D (ID INT PRIMARY KEY IDENTITY, C1 INT, C2 INT)
INSERT D VALUES (3,NULL),
(5,1),
(5,1),
(5,1),
(4,NULL),
(3,NULL),
(5,2),
(5,1),
(5,2),
(5,2)
The query produces the output indicated above:
ID C1 C2 LOT
1 3
2 5 1 1
3 5 1 2
4 5 1 3
5 4
6 3
7 5 2 1
8 5 1 4
9 5 2 2
10 5 2 3
The statement used to generate LOT, COUNT(*) OVER (PARTITION BY C1, C2 ORDER BY ID ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, simply counts the number of rows before and including the current row where C1 and C2 are equal to the current row. For instance, for Row 4, the tuple (c1,c2)=(5,1) is observed in 3 records before and including that row (rows 2, 3, and 4), so LOT=3.

Answer 4

Thank you everyone for the suggestions to use a trigger (all up-voted). It turns out (as I mentioned in a comment above) an article that came up on the side-bar (SQL Server unique auto-increment column in the context of another column) shows a detailed construction of the proper INSTEAD OF INSERT trigger. The author mentions that it is "Not tested", and indeed there IS a slight error (a missing GROUP BY ParentEntityID in the WITH clause) but anybody copying the code would get an error that is obvious to fix. Probably not kosher to be correcting that post here, but the other question is 6 years old.