问题
I want to generate a custom identity column related to type of product. Can this query guaranty the order of identity and resolve concurrency. This is a sample query:
BEGIN TRAN INSERT INTO TBLKEY VALUES((SELECT 'A-' + CAST(MAX(CAST(ID AS INT)) + 1 AS NVARCHAR) FROM TBLKEY),'EHSAN') COMMIT
回答1:
Try this:
BEGIN TRAN
INSERT INTO TBLKEY
VALUES((SELECT MAX(ID) + 1 AS NVARCHAR) FROM TBLKEY WITH (UPDLOCK)),'EHSAN')
COMMIT
When selecting the max ID you acquire a U lock on the row. The U lock is incompatible with the U lock which will try to acquire another session with the same query running at the same time. Only one query will be executed at a given time. The ids will be in order and continuous without any gaps between them.
A better solution would be to create an extra table dedicated only for storing the current or next id and use it instead of the maximum.
You can understand the difference by doing the following:
Prepare a table
CREATE TABLE T(id int not null PRIMARY KEY CLUSTERED)
INSERT INTO T VALUES(1)
And then run the following query in two different sessions one after another with less than 10 seconds apart
BEGIN TRAN
DECLARE @idv int
SELECT @idv = max (id) FROM T
WAITFOR DELAY '0:0:10'
INSERT INTO T VALUES(@idv+1)
COMMIT
Wait for a while until both queries complete. Observe that one of them succeeded and the other failed.
Now do the same with the following query
BEGIN TRAN
DECLARE @idv int
SELECT @idv = max (id) FROM T WITH (UPDLOCK)
WAITFOR DELAY '0:0:5'
INSERT INTO T VALUES(@idv+1)
COMMIT
View the contents of T
Cleanup the T Table with DROP TABLE T
回答2:
This would be a bad thing to do as there is no way to guarantee that two queries running at the same time wouldn't get MAX(ID) as being the same value.
If you used a standard identity column you could also have a computed column which uses that or just return the key when you return the data.
Ed
来源：https://stackoverflow.com/questions/29937401/sql-server-custom-identity-column