Get IDENTITY value in the same T-SQL statement it is created in?

问题

I was asked if you could have an insert statement, which had an ID field that was an "identity" column, and if the value that was assigned could also be inserted into another field in the same record, in the same insert statement.

Is this possible (SQL Server 2008r2)?

Thanks.

回答1:

You cannot really do this - because the actual value that will be used for the IDENTITY column really only is fixed and set when the INSERT has completed.

You could however use e.g. a trigger

CREATE TRIGGER trg_YourTableInsertID ON dbo.YourTable 
AFTER INSERT
AS 
UPDATE dbo.YourTable
SET dbo.YourTable.OtherID = i.ID
FROM dbo.YourTable t2
INNER JOIN INSERTED i ON i.ID = t2.ID

This would fire right after any rows have been inserted, and would set the OtherID column to the values of the IDENTITY columns for the inserted rows. But it's strictly speaking not within the same statement - it's just after your original statement.

回答2:

You can do this by having a computed column in your table:

 DECLARE    @QQ TABLE (ID INT IDENTITY(1,1), Computed AS ID PERSISTED, Letter VARCHAR (1))

INSERT INTO @QQ (Letter)
VALUES ('h'),
('e'),
('l'),
('l'),
('o')

SELECT  *
FROM    @QQ

1   1   h

2   2   e

3   3   l

4   4   l

5   5   o

回答3:

About the cheked answer:

You cannot really do this - because the actual value that will be used for the IDENTITY column really only is fixed and set when the INSERT has completed.

marc_s I suppose, you are not actually right. Yes, He can! ))

The way to solution is IDENT_CURRENT():

CREATE TABLE TemporaryTable(
    Id int PRIMARY KEY IDENTITY(1,1) NOT NULL,
    FkId int NOT NULL 
)

ALTER TABLE TemporaryTable
    ADD CONSTRAINT [Fk_const] FOREIGN KEY (FkId) REFERENCES [TemporaryTable] ([Id])

INSERT INTO TemporaryTable (FkId) VALUES  (IDENT_CURRENT('[TemporaryTable]'))
INSERT INTO TemporaryTable (FkId) VALUES  (IDENT_CURRENT('[TemporaryTable]'))
INSERT INTO TemporaryTable (FkId) VALUES  (IDENT_CURRENT('[TemporaryTable]'))
INSERT INTO TemporaryTable (FkId) VALUES  (IDENT_CURRENT('[TemporaryTable]'))

UPDATE TemporaryTable 
   SET [FkId] = 3
 WHERE Id = 2

SELECT * FROM TemporaryTable

DROP TABLE TemporaryTable

More over, you can even use IDENT_CURRENT() as DEFAULT CONSTRAINT and it works instead of SCOPE_IDENTITY() for example. Try this:

CREATE TABLE TemporaryTable(
    Id int PRIMARY KEY IDENTITY(1,1) NOT NULL,
    FkId int NOT NULL DEFAULT IDENT_CURRENT('[TemporaryTable]')
)

ALTER TABLE TemporaryTable
    ADD CONSTRAINT [Fk_const] FOREIGN KEY (FkId) REFERENCES [TemporaryTable] ([Id])

INSERT INTO TemporaryTable (FkId) VALUES (DEFAULT)
INSERT INTO TemporaryTable (FkId) VALUES (DEFAULT)
INSERT INTO TemporaryTable (FkId) VALUES (DEFAULT)
INSERT INTO TemporaryTable (FkId) VALUES (DEFAULT)

UPDATE TemporaryTable 
   SET [FkId] = 3
 WHERE Id = 2

SELECT * FROM TemporaryTable

DROP TABLE TemporaryTable

回答4:

You can do both.

To insert rows with a column "identity", you need to set identity_insert off.

Note that you still can't duplicate values!

You can see the command here. Be aware to set identity_insert on afterwards.

To create a table with the same record, you simply need to:

create new column;
insert it with null value or other thing;
update that column after inserts with the value of the identity column.

If you need to insert the value at the same time, you can use the @@identity global variable. It'll give you the last inserted. So I think you need to do a @@identity + 1. In this case it can give wrong values because the @@identity is for all tables. So it'll count if the insert occurs in another table with identity.

Another solution is to get the max id and add one :) and you get the needed value!