Use a sequence to populate multiple columns with successive values - tsql

Question

How can I use default constraints, triggers, or some other mechanism to automatically insert multiple successive values from a sequence into multiple columns on the same row of a table?
A standard use of a sequence in SQL Server is to combine it with default constraints on multiple tables to essentially get a cross-table identity. See for example the section "C. Using a Sequence Number in Multiple Tables" in the Microsoft documentation article "Sequence Numbers".
This works great if you only want to get a single value from the sequence for each row inserted. But sometimes I want to get multiple successive values. So theoretically I would create a sequence and table like this:
CREATE SEQUENCE DocumentationIDs;
CREATE TABLE Product
(
ProductID BIGINT NOT NULL IDENTITY(1, 1) PRIMARY KEY
, ProductName NVARCHAR(100) NOT NULL
, MarketingDocumentationID BIGINT NOT NULL DEFAULT ( NEXT VALUE FOR DocumentationIDs )
, TechnicalDocumentationID BIGINT NOT NULL DEFAULT ( NEXT VALUE FOR DocumentationIDs )
, InternalDocumentationID BIGINT NOT NULL DEFAULT ( NEXT VALUE FOR DocumentationIDs )
);
Unfortunately this will insert the same value in all three columns. This is by design:
If there are multiple instances of the NEXT VALUE FOR function specifying the same sequence generator within a single Transact-SQL statement, all those instances return the same value for a given row processed by that Transact-SQL statement. This behavior is consistent with the ANSI standard.
Increment by hack
The only suggestion I could find online was to use a hack where you have the sequence increment by the number of columns you need to insert (three in my contrived example) and manually add to the NEXT VALUE FOR function in the default constraint:
CREATE SEQUENCE DocumentationIDs START WITH 1 INCREMENT BY 3;
CREATE TABLE Product
(
ProductID BIGINT NOT NULL IDENTITY(1, 1) PRIMARY KEY
, ProductName NVARCHAR(100) NOT NULL
, MarketingDocumentationID BIGINT NOT NULL DEFAULT ( NEXT VALUE FOR DocumentationIDs )
, TechnicalDocumentationID BIGINT NOT NULL DEFAULT ( ( NEXT VALUE FOR DocumentationIDs ) + 1 )
, InternalDocumentationID BIGINT NOT NULL DEFAULT ( ( NEXT VALUE FOR DocumentationIDs ) + 2 )
)
This does not work for me because not all tables using my sequence require the same number of values.

Answer 1

One possible way using AFTER INSERT trigger is following.
Table definition need to be changed slighlty (DocumentationID columns should be defaulted to 0, or allowed to be nullable):
CREATE TABLE Product
(
ProductID BIGINT NOT NULL IDENTITY(1, 1)
, ProductName NVARCHAR(100) NOT NULL
, MarketingDocumentationID BIGINT NOT NULL CONSTRAINT DF_Product_1 DEFAULT (0)
, TechnicalDocumentationID BIGINT NOT NULL CONSTRAINT DF_Product_2 DEFAULT (0)
, InternalDocumentationID BIGINT NOT NULL CONSTRAINT DF_Product_3 DEFAULT (0)
, CONSTRAINT PK_Product PRIMARY KEY (ProductID)
);
And the trigger doing the job is following:
CREATE TRIGGER Product_AfterInsert ON Product
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON;
IF NOT EXISTS (SELECT 1 FROM INSERTED)
RETURN;
CREATE TABLE #DocIDs
(
ProductID BIGINT NOT NULL
, Num INT NOT NULL
, DocID BIGINT NOT NULL
, PRIMARY KEY (ProductID, Num)
);
INSERT INTO #DocIDs (ProductID, Num, DocID)
SELECT
i.ProductID
, r.n
, NEXT VALUE FOR DocumentationIDs OVER (ORDER BY i.ProductID, r.n)
FROM INSERTED i
CROSS APPLY (VALUES (1), (2), (3)) r(n)
;
WITH Docs (ProductID, MarketingDocID, TechnicalDocID, InternalDocID)
AS (
SELECT ProductID, [1], [2], [3]
FROM #DocIDs d
PIVOT (MAX(DocID) FOR Num IN ([1], [2], [3])) pvt
)
UPDATE p
SET
p.MarketingDocumentationID = d.MarketingDocID
, p.TechnicalDocumentationID = d.TechnicalDocID
, p.InternalDocumentationID = d.InternalDocID
FROM Product p
JOIN Docs d ON d.ProductID = p.ProductID
;
END