Identity field between 2 tables

Question

I have 2 tables which should have ID as identity column in common.
For example:
If there is insert insert into first table, the id is 1.
Then there is insert into second table, the id is 2(and not 1).
Then insert into first table, the id should be 3 (and not 2).
What is the best way to do that?
One is to have identity and use reseed every time.
Thanks,
Simon

Answer 1

If you are on SQL 2012, count yourself lucky. You should use sequences, period.
You will have to create new table definitions and copy data over for T1, if T1 has an IDENTITY column to day. (You can't drop the IDENTITY property with ALTER TABLE.) You set the start value for the sequence to be ident_current of that table + 1.
The simplest way to use the sequence is in a default, as in the quick demo below:
CREATE SEQUENCE MySeq AS int START WITH 1
go
CREATE TABLE albin (a int NOT NULL
CONSTRAINT def_albin_a DEFAULT NEXT VALUE FOR MySeq,
somemoredata varchar(132) NOT NULL,
CONSTRAINT pk_albin PRIMARY KEY (a)
CREATE TABLE beata (b int NOT NULL
CONSTRAINT def_beata_b DEFAULT NEXT VALUE FOR MySeq,
someotherdata varchar(132) NOT NULL,
CONSTRAINT pk_beata PRIMARY KEY (b)
go
INSERT albin (somemoredata)
OUTPUT inserted.a
VALUES ('somemoredata')
INSERT beata (someotherdata)
OUTPUT inserted.b
VALUES ('someotherdata')
go
DROP TABLE albin
DROP TABLE beata
DROP SEQUENCE MySeq
Erland Sommarskog, SQL Server MVP, [email protected]