Related
I have stored procedure in the sql server 2008, my stored procedure calculate and get the last number "not primary key" from column from table B and add one ( +1 ) to this number to use it on the next statement on the same stored procedure.
My issue that i have a duplicate number some times, i think this happened when multiple users call the stored procedure on the same time. is this the issue and how can i solve it
my code is like the below:-
DECLARE #ID AS NVARCHAR(10)
SET #ID = (
SELECT TOP 1 MyNo
FROM Employee
WHERE (
(TypeID = #TypeID) AND
(Year = #Year)
)
ORDER BY ID DESC
)
SET #ID = ISNULL(#ID,0) + 1
INSERT INTO Employee (name,lname,MyNo) VALUES (#name,#lname,#MyNo)
You can lock a table for the duration of a transaction with the WITH (TABLOCKX, HOLDLOCK) syntax:
BEGIN TRANSACTION
DECLARE #ID AS NVARCHAR(10)
SET #ID = (
SELECT TOP 1 MyNo
FROM Employee WITH (TABLOCKX, HOLDLOCK)
WHERE (
(TypeID = #TypeID) AND
(Year = #Year)
)
ORDER BY ID DESC
)
SET #ID = ISNULL(#ID,0) + 1
INSERT INTO Employee (name,lname,MyNo) VALUES (#name,#lname,#MyNo)
COMMIT TRANSACTION
You can find more information about TABLOCK and TABLOCKX here:
https://learn.microsoft.com/en-us/sql/t-sql/queries/hints-transact-sql-table
Per discussion, the best lock to use in this case would be:
(UPDLOCK,HOLDLOCK)
If you cannot use Identity column or the Table lock, another alternative is to use sp_getapplock
The advantage with this mechanism is that this kind of lock can be used across multiple stored procedures that should not run concurrently or for operations that span multiple tables. It also allows for handling timeout and other kinds of behavior if the lock is not available.
You have to be careful when using this feature and ensure you acquire and release locks properly or you will create more problems than you solve.
Assume a table structure of MyTable(KEY, datafield1, datafield2...).
Often I want to either update an existing record, or insert a new record if it doesn't exist.
Essentially:
IF (key exists)
run update command
ELSE
run insert command
What's the best performing way to write this?
don't forget about transactions. Performance is good, but simple (IF EXISTS..) approach is very dangerous.
When multiple threads will try to perform Insert-or-update you can easily
get primary key violation.
Solutions provided by #Beau Crawford & #Esteban show general idea but error-prone.
To avoid deadlocks and PK violations you can use something like this:
begin tran
if exists (select * from table with (updlock,serializable) where key = #key)
begin
update table set ...
where key = #key
end
else
begin
insert into table (key, ...)
values (#key, ...)
end
commit tran
or
begin tran
update table with (serializable) set ...
where key = #key
if ##rowcount = 0
begin
insert into table (key, ...) values (#key,..)
end
commit tran
See my detailed answer to a very similar previous question
#Beau Crawford's is a good way in SQL 2005 and below, though if you're granting rep it should go to the first guy to SO it. The only problem is that for inserts it's still two IO operations.
MS Sql2008 introduces merge from the SQL:2003 standard:
merge tablename with(HOLDLOCK) as target
using (values ('new value', 'different value'))
as source (field1, field2)
on target.idfield = 7
when matched then
update
set field1 = source.field1,
field2 = source.field2,
...
when not matched then
insert ( idfield, field1, field2, ... )
values ( 7, source.field1, source.field2, ... )
Now it's really just one IO operation, but awful code :-(
Do an UPSERT:
UPDATE MyTable SET FieldA=#FieldA WHERE Key=#Key
IF ##ROWCOUNT = 0
INSERT INTO MyTable (FieldA) VALUES (#FieldA)
http://en.wikipedia.org/wiki/Upsert
Many people will suggest you use MERGE, but I caution you against it. By default, it doesn't protect you from concurrency and race conditions any more than multiple statements, and it introduces other dangers:
Use Caution with SQL Server's MERGE Statement
So, you want to use MERGE, eh?
Even with this "simpler" syntax available, I still prefer this approach (error handling omitted for brevity):
BEGIN TRANSACTION;
UPDATE dbo.table WITH (UPDLOCK, SERIALIZABLE)
SET ... WHERE PK = #PK;
IF ##ROWCOUNT = 0
BEGIN
INSERT dbo.table(PK, ...) SELECT #PK, ...;
END
COMMIT TRANSACTION;
Please stop using this UPSERT anti-pattern
A lot of folks will suggest this way:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
IF EXISTS (SELECT 1 FROM dbo.table WHERE PK = #PK)
BEGIN
UPDATE ...
END
ELSE
BEGIN
INSERT ...
END
COMMIT TRANSACTION;
But all this accomplishes is ensuring you may need to read the table twice to locate the row(s) to be updated. In the first sample, you will only ever need to locate the row(s) once. (In both cases, if no rows are found from the initial read, an insert occurs.)
Others will suggest this way:
BEGIN TRY
INSERT ...
END TRY
BEGIN CATCH
IF ERROR_NUMBER() = 2627
UPDATE ...
END CATCH
However, this is problematic if for no other reason than letting SQL Server catch exceptions that you could have prevented in the first place is much more expensive, except in the rare scenario where almost every insert fails. I prove as much here:
Checking for potential constraint violations before entering TRY/CATCH
Performance impact of different error handling techniques
IF EXISTS (SELECT * FROM [Table] WHERE ID = rowID)
UPDATE [Table] SET propertyOne = propOne, property2 . . .
ELSE
INSERT INTO [Table] (propOne, propTwo . . .)
Edit:
Alas, even to my own detriment, I must admit the solutions that do this without a select seem to be better since they accomplish the task with one less step.
If you want to UPSERT more than one record at a time you can use the ANSI SQL:2003 DML statement MERGE.
MERGE INTO table_name WITH (HOLDLOCK) USING table_name ON (condition)
WHEN MATCHED THEN UPDATE SET column1 = value1 [, column2 = value2 ...]
WHEN NOT MATCHED THEN INSERT (column1 [, column2 ...]) VALUES (value1 [, value2 ...])
Check out Mimicking MERGE Statement in SQL Server 2005.
Although its pretty late to comment on this I want to add a more complete example using MERGE.
Such Insert+Update statements are usually called "Upsert" statements and can be implemented using MERGE in SQL Server.
A very good example is given here:
http://weblogs.sqlteam.com/dang/archive/2009/01/31/UPSERT-Race-Condition-With-MERGE.aspx
The above explains locking and concurrency scenarios as well.
I will be quoting the same for reference:
ALTER PROCEDURE dbo.Merge_Foo2
#ID int
AS
SET NOCOUNT, XACT_ABORT ON;
MERGE dbo.Foo2 WITH (HOLDLOCK) AS f
USING (SELECT #ID AS ID) AS new_foo
ON f.ID = new_foo.ID
WHEN MATCHED THEN
UPDATE
SET f.UpdateSpid = ##SPID,
UpdateTime = SYSDATETIME()
WHEN NOT MATCHED THEN
INSERT
(
ID,
InsertSpid,
InsertTime
)
VALUES
(
new_foo.ID,
##SPID,
SYSDATETIME()
);
RETURN ##ERROR;
/*
CREATE TABLE ApplicationsDesSocietes (
id INT IDENTITY(0,1) NOT NULL,
applicationId INT NOT NULL,
societeId INT NOT NULL,
suppression BIT NULL,
CONSTRAINT PK_APPLICATIONSDESSOCIETES PRIMARY KEY (id)
)
GO
--*/
DECLARE #applicationId INT = 81, #societeId INT = 43, #suppression BIT = 0
MERGE dbo.ApplicationsDesSocietes WITH (HOLDLOCK) AS target
--set the SOURCE table one row
USING (VALUES (#applicationId, #societeId, #suppression))
AS source (applicationId, societeId, suppression)
--here goes the ON join condition
ON target.applicationId = source.applicationId and target.societeId = source.societeId
WHEN MATCHED THEN
UPDATE
--place your list of SET here
SET target.suppression = source.suppression
WHEN NOT MATCHED THEN
--insert a new line with the SOURCE table one row
INSERT (applicationId, societeId, suppression)
VALUES (source.applicationId, source.societeId, source.suppression);
GO
Replace table and field names by whatever you need.
Take care of the using ON condition.
Then set the appropriate value (and type) for the variables on the DECLARE line.
Cheers.
That depends on the usage pattern. One has to look at the usage big picture without getting lost in the details. For example, if the usage pattern is 99% updates after the record has been created, then the 'UPSERT' is the best solution.
After the first insert (hit), it will be all single statement updates, no ifs or buts. The 'where' condition on the insert is necessary otherwise it will insert duplicates, and you don't want to deal with locking.
UPDATE <tableName> SET <field>=#field WHERE key=#key;
IF ##ROWCOUNT = 0
BEGIN
INSERT INTO <tableName> (field)
SELECT #field
WHERE NOT EXISTS (select * from tableName where key = #key);
END
You can use MERGE Statement, This statement is used to insert data if not exist or update if does exist.
MERGE INTO Employee AS e
using EmployeeUpdate AS eu
ON e.EmployeeID = eu.EmployeeID`
If going the UPDATE if-no-rows-updated then INSERT route, consider doing the INSERT first to prevent a race condition (assuming no intervening DELETE)
INSERT INTO MyTable (Key, FieldA)
SELECT #Key, #FieldA
WHERE NOT EXISTS
(
SELECT *
FROM MyTable
WHERE Key = #Key
)
IF ##ROWCOUNT = 0
BEGIN
UPDATE MyTable
SET FieldA=#FieldA
WHERE Key=#Key
IF ##ROWCOUNT = 0
... record was deleted, consider looping to re-run the INSERT, or RAISERROR ...
END
Apart from avoiding a race condition, if in most cases the record will already exist then this will cause the INSERT to fail, wasting CPU.
Using MERGE probably preferable for SQL2008 onwards.
MS SQL Server 2008 introduces the MERGE statement, which I believe is part of the SQL:2003 standard. As many have shown it is not a big deal to handle one row cases, but when dealing with large datasets, one needs a cursor, with all the performance problems that come along. The MERGE statement will be much welcomed addition when dealing with large datasets.
Before everyone jumps to HOLDLOCK-s out of fear from these nafarious users running your sprocs directly :-) let me point out that you have to guarantee uniqueness of new PK-s by design (identity keys, sequence generators in Oracle, unique indexes for external ID-s, queries covered by indexes). That's the alpha and omega of the issue. If you don't have that, no HOLDLOCK-s of the universe are going to save you and if you do have that then you don't need anything beyond UPDLOCK on the first select (or to use update first).
Sprocs normally run under very controlled conditions and with the assumption of a trusted caller (mid tier). Meaning that if a simple upsert pattern (update+insert or merge) ever sees duplicate PK that means a bug in your mid-tier or table design and it's good that SQL will yell a fault in such case and reject the record. Placing a HOLDLOCK in this case equals eating exceptions and taking in potentially faulty data, besides reducing your perf.
Having said that, Using MERGE, or UPDATE then INSERT is easier on your server and less error prone since you don't have to remember to add (UPDLOCK) to first select. Also, if you are doing inserts/updates in small batches you need to know your data in order to decide whether a transaction is appropriate or not. It it's just a collection of unrelated records then additional "enveloping" transaction will be detrimental.
Does the race conditions really matter if you first try an update followed by an insert?
Lets say you have two threads that want to set a value for key key:
Thread 1: value = 1
Thread 2: value = 2
Example race condition scenario
key is not defined
Thread 1 fails with update
Thread 2 fails with update
Exactly one of thread 1 or thread 2 succeeds with insert. E.g. thread 1
The other thread fails with insert (with error duplicate key) - thread 2.
Result: The "first" of the two treads to insert, decides value.
Wanted result: The last of the 2 threads to write data (update or insert) should decide value
But; in a multithreaded environment, the OS scheduler decides on the order of the thread execution - in the above scenario, where we have this race condition, it was the OS that decided on the sequence of execution. Ie: It is wrong to say that "thread 1" or "thread 2" was "first" from a system viewpoint.
When the time of execution is so close for thread 1 and thread 2, the outcome of the race condition doesn't matter. The only requirement should be that one of the threads should define the resulting value.
For the implementation: If update followed by insert results in error "duplicate key", this should be treated as success.
Also, one should of course never assume that value in the database is the same as the value you wrote last.
I had tried below solution and it works for me, when concurrent request for insert statement occurs.
begin tran
if exists (select * from table with (updlock,serializable) where key = #key)
begin
update table set ...
where key = #key
end
else
begin
insert table (key, ...)
values (#key, ...)
end
commit tran
You can use this query. Work in all SQL Server editions. It's simple, and clear. But you need use 2 queries. You can use if you can't use MERGE
BEGIN TRAN
UPDATE table
SET Id = #ID, Description = #Description
WHERE Id = #Id
INSERT INTO table(Id, Description)
SELECT #Id, #Description
WHERE NOT EXISTS (SELECT NULL FROM table WHERE Id = #Id)
COMMIT TRAN
NOTE: Please explain answer negatives
Assuming that you want to insert/update single row, most optimal approach is to use SQL Server's REPEATABLE READ transaction isolation level:
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
BEGIN TRANSACTION
IF (EXISTS (SELECT * FROM myTable WHERE key=#key)
UPDATE myTable SET ...
WHERE key=#key
ELSE
INSERT INTO myTable (key, ...)
VALUES (#key, ...)
COMMIT TRANSACTION
This isolation level will prevent/block subsequent repeatable read transactions from accessing same row (WHERE key=#key) while currently running transaction is open.
On the other hand, operations on another row won't be blocked (WHERE key=#key2).
You can use:
INSERT INTO tableName (...) VALUES (...)
ON DUPLICATE KEY
UPDATE ...
Using this, if there is already an entry for the particular key, then it will UPDATE, else, it will INSERT.
In SQL Server 2008 you can use the MERGE statement
If you use ADO.NET, the DataAdapter handles this.
If you want to handle it yourself, this is the way:
Make sure there is a primary key constraint on your key column.
Then you:
Do the update
If the update fails because a record with the key already exists, do the insert. If the update does not fail, you are finished.
You can also do it the other way round, i.e. do the insert first, and do the update if the insert fails. Normally the first way is better, because updates are done more often than inserts.
Doing an if exists ... else ... involves doing two requests minimum (one to check, one to take action). The following approach requires only one where the record exists, two if an insert is required:
DECLARE #RowExists bit
SET #RowExists = 0
UPDATE MyTable SET DataField1 = 'xxx', #RowExists = 1 WHERE Key = 123
IF #RowExists = 0
INSERT INTO MyTable (Key, DataField1) VALUES (123, 'xxx')
I usually do what several of the other posters have said with regard to checking for it existing first and then doing whatever the correct path is. One thing you should remember when doing this is that the execution plan cached by sql could be nonoptimal for one path or the other. I believe the best way to do this is to call two different stored procedures.
FirstSP:
If Exists
Call SecondSP (UpdateProc)
Else
Call ThirdSP (InsertProc)
Now, I don't follow my own advice very often, so take it with a grain of salt.
Do a select, if you get a result, update it, if not, create it.
I need to update a column of all rows of a table and I need to use UPDLOCK to do it.
For example:
UPDATE table (UPDLock)
SET column_name = ‘123’
Another alternative is to use an for update cursor and update each row. The advantage with the second approach is that the lock is not held till the end of the transaction and concurrent updates of the same rows can happen sooner. At the same time update cursors are said to have bad performance. Which is a better approach?
EDIT:
Assume the column is updated with a value that is derived from another column in the table. In other words, column_name = f(column_name_1)
You cannot give an UPDLOCK hint to a write operation, like UPDATE statement. It will be ignored, since all writes (INSERT/UPDATE/DELETE) take the same lock, an exclusive lock on the row being updated. You can quickly validate this yourself:
create table heap (a int);
go
insert into heap (a) values (1)
go
begin transaction
update heap
--with (UPDLOCK)
set a=2
select * from sys.dm_tran_locks
rollback
If you remove the comment -- on the with (UPDLOCK) you'll see that you get excatly the same locks (an X lock on the physical row). You can do the same experiment with a B-Tree instead of a heap:
create table btree (a int not null identity(1,1) primary key, b int)
go
insert into btree (b) values (1)
go
begin transaction
update btree
--with (UPDLOCK)
set b=2
select * from sys.dm_tran_locks
rollback
Again, the locks acquired will be identical with or w/o the hint (an exclusive lock on the row key).
Now back to your question, can this whole table update be done in batches? (since this is basically what you're asking). Yes, if the table has a primary key (to be precise what's required is an unique index to batch on, preferable the clustered index to avoid tipping point issues). Here is an example how:
create table btree (id int not null identity(1,1) primary key, b int, c int);
go
set nocount on;
insert into btree (b) values (rand()*1000);
go 1000
declare #id int = null, #rc int;
declare #inserted table (id int);
begin transaction;
-- first batch has no WHERE clause
with cte as (
select top(10) id, b, c
from btree
order by id)
update cte
set c = b+1
output INSERTED.id into #inserted (id);
set #rc = ##rowcount;
commit;
select #id = max(id) from #inserted;
delete from #inserted;
raiserror (N'Updated %d rows, up to id %d', 0,0,#rc, #id);
begin transaction;
while (1=1)
begin
-- update the next batch of 10 rows, now it has where clause
with cte as (
select top(10) id, b, c
from btree
where id > #id
order by id)
update cte
set c = b+1
output INSERTED.id into #inserted (id);
set #rc = ##rowcount;
if (0 = #rc)
break;
commit;
begin transaction;
select #id = max(id) from #inserted;
delete from #inserted;
raiserror (N'Updated %d rows, up to id %d', 0,0,#rc, #id);
end
commit
go
If your table doesn't have a unique clustered index then it becomes really tricky to do this, you would need to do the same thing a cursor has to do. While from a logical point of view the index is not required, not having it would cause each batch to do a whole-table-scan, which would be pretty much disastrous.
In case you wonder what happens if someone inserts a value behind the current #id, then the answer is very simple: the exactly same thing that would happen if someone inserts a value after the whole processing is complete.
Personally I think the single UPDATE will be much better. There are very few cases where a cursor will be better overall, regardless of concurrent activity. In fact the only one that comes to mind is a very complex running totals query - I don't think I've ever seen better overall performance from a cursor that is not read only, only SELECT queries. Of course, you have much better means of testing which is "a better approach" - you have your hardware, your schema, your data, and your usage patterns right in front of you. All you have to do is perform some tests.
That all said, what is the point in the first place of updating that column so that every single row has the same value? I suspect that if the value in that column has no bearing to the rest of the row, it can be stored elsewhere - perhaps a related table or a single-row table. Maybe the value in that column should be NULL (in which case you get it from the other table) unless it is overriden for a specific row. It seems to me like there is a better solution here than touching every single row in the table every time.
I have a primary key that I don't want to auto increment (for various reasons) and so I'm looking for a way to simply increment that field when I INSERT. By simply, I mean without stored procedures and without triggers, so just a series of SQL commands (preferably one command).
Here is what I have tried thus far:
BEGIN TRAN
INSERT INTO Table1(id, data_field)
VALUES ( (SELECT (MAX(id) + 1) FROM Table1), '[blob of data]');
COMMIT TRAN;
* Data abstracted to use generic names and identifiers
However, when executed, the command errors, saying that
"Subqueries are not allowed in this
context. only scalar expressions are
allowed"
So, how can I do this/what am I doing wrong?
EDIT: Since it was pointed out as a consideration, the table to be inserted into is guaranteed to have at least 1 row already.
You understand that you will have collisions right?
you need to do something like this and this might cause deadlocks so be very sure what you are trying to accomplish here
DECLARE #id int
BEGIN TRAN
SELECT #id = MAX(id) + 1 FROM Table1 WITH (UPDLOCK, HOLDLOCK)
INSERT INTO Table1(id, data_field)
VALUES (#id ,'[blob of data]')
COMMIT TRAN
To explain the collision thing, I have provided some code
first create this table and insert one row
CREATE TABLE Table1(id int primary key not null, data_field char(100))
GO
Insert Table1 values(1,'[blob of data]')
Go
Now open up two query windows and run this at the same time
declare #i int
set #i =1
while #i < 10000
begin
BEGIN TRAN
INSERT INTO Table1(id, data_field)
SELECT MAX(id) + 1, '[blob of data]' FROM Table1
COMMIT TRAN;
set #i =#i + 1
end
You will see a bunch of these
Server: Msg 2627, Level 14, State 1, Line 7
Violation of PRIMARY KEY constraint 'PK__Table1__3213E83F2962141D'. Cannot insert duplicate key in object 'dbo.Table1'.
The statement has been terminated.
Try this instead:
INSERT INTO Table1 (id, data_field)
SELECT id, '[blob of data]' FROM (SELECT MAX(id) + 1 as id FROM Table1) tbl
I wouldn't recommend doing it that way for any number of reasons though (performance, transaction safety, etc)
It could be because there are no records so the sub query is returning NULL...try
INSERT INTO tblTest(RecordID, Text)
VALUES ((SELECT ISNULL(MAX(RecordID), 0) + 1 FROM tblTest), 'asdf')
I don't know if somebody is still looking for an answer but here is a solution that seems to work:
-- Preparation: execute only once
CREATE TABLE Test (Value int)
CREATE TABLE Lock (LockID uniqueidentifier)
INSERT INTO Lock SELECT NEWID()
-- Real insert
BEGIN TRAN LockTran
-- Lock an object to block simultaneous calls.
UPDATE Lock WITH(TABLOCK)
SET LockID = LockID
INSERT INTO Test
SELECT ISNULL(MAX(T.Value), 0) + 1
FROM Test T
COMMIT TRAN LockTran
We have a similar situation where we needed to increment and could not have gaps in the numbers. (If you use an identity value and a transaction is rolled back, that number will not be inserted and you will have gaps because the identity value does not roll back.)
We created a separate table for last number used and seeded it with 0.
Our insert takes a few steps.
--increment the number
Update dbo.NumberTable
set number = number + 1
--find out what the incremented number is
select #number = number
from dbo.NumberTable
--use the number
insert into dbo.MyTable using the #number
commit or rollback
This causes simultaneous transactions to process in a single line as each concurrent transaction will wait because the NumberTable is locked. As soon as the waiting transaction gets the lock, it increments the current value and locks it from others. That current value is the last number used and if a transaction is rolled back, the NumberTable update is also rolled back so there are no gaps.
Hope that helps.
Another way to cause single file execution is to use a SQL application lock. We have used that approach for longer running processes like synchronizing data between systems so only one synchronizing process can run at a time.
If you're doing it in a trigger, you could make sure it's an "INSTEAD OF" trigger and do it in a couple of statements:
DECLARE #next INT
SET #next = (SELECT (MAX(id) + 1) FROM Table1)
INSERT INTO Table1
VALUES (#next, inserted.datablob)
The only thing you'd have to be careful about is concurrency - if two rows are inserted at the same time, they could attempt to use the same value for #next, causing a conflict.
Does this accomplish what you want?
It seems very odd to do this sort of thing w/o an IDENTITY (auto-increment) column, making me question the architecture itself. I mean, seriously, this is the perfect situation for an IDENTITY column. It might help us answer your question if you'd explain the reasoning behind this decision. =)
Having said that, some options are:
using an INSTEAD OF trigger for this purpose. So, you'd do your INSERT (the INSERT statement would not need to pass in an ID). The trigger code would handle inserting the appropriate ID. You'd need to use the WITH (UPDLOCK, HOLDLOCK) syntax used by another answerer to hold the lock for the duration of the trigger (which is implicitly wrapped in a transaction) & to elevate the lock type from "shared" to "update" lock (IIRC).
you can use the idea above, but have a table whose purpose is to store the last, max value inserted into the table. So, once the table is set up, you would no longer have to do a SELECT MAX(ID) every time. You'd simply increment the value in the table. This is safe provided that you use appropriate locking (as discussed). Again, that avoids repeated table scans every time you INSERT.
use GUIDs instead of IDs. It's much easier to merge tables across databases, since the GUIDs will always be unique (whereas records across databases will have conflicting integer IDs). To avoid page splitting, sequential GUIDs can be used. This is only beneficial if you might need to do database merging.
Use a stored proc in lieu of the trigger approach (since triggers are to be avoided, for some reason). You'd still have the locking issue (and the performance problems that can arise). But sprocs are preferred over dynamic SQL (in the context of applications), and are often much more performant.
Sorry about rambling. Hope that helps.
How about creating a separate table to maintain the counter? It has better performance than MAX(id), as it will be O(1). MAX(id) is at best O(lgn) depending on the implementation.
And then when you need to insert, simply lock the counter table for reading the counter and increment the counter. Then you can release the lock and insert to your table with the incremented counter value.
Have a separate table where you keep your latest ID and for every transaction get a new one.
It may be a bit slower but it should work.
DECLARE #NEWID INT
BEGIN TRAN
UPDATE TABLE SET ID=ID+1
SELECT #NEWID=ID FROM TABLE
COMMIT TRAN
PRINT #NEWID -- Do what you want with your new ID
Code without any transaction scope (I use it in my engineer course as an exercice) :
-- Preparation: execute only once
CREATE TABLE increment (val int);
INSERT INTO increment VALUES (1);
-- Real insert
DECLARE #newIncrement INT;
UPDATE increment
SET #newIncrement = val,
val = val + 1;
INSERT INTO Table1 (id, data_field)
SELECT #newIncrement, 'some data';
declare #nextId int
set #nextId = (select MAX(id)+1 from Table1)
insert into Table1(id, data_field) values (#nextId, '[blob of data]')
commit;
But perhaps a better approach would be using a scalar function getNextId('table1')
Any critiques of this? Works for me.
DECLARE #m_NewRequestID INT
, #m_IsError BIT = 1
, #m_CatchEndless INT = 0
WHILE #m_IsError = 1
BEGIN TRY
SELECT #m_NewRequestID = (SELECT ISNULL(MAX(RequestID), 0) + 1 FROM Requests)
INSERT INTO Requests ( RequestID
, RequestName
, Customer
, Comment
, CreatedFromApplication)
SELECT RequestID = #m_NewRequestID
, RequestName = dbo.ufGetNextAvailableRequestName(PatternName)
, Customer = #Customer
, Comment = [Description]
, CreatedFromApplication = #CreatedFromApplication
FROM RequestPatterns
WHERE PatternID = #PatternID
SET #m_IsError = 0
END TRY
BEGIN CATCH
SET #m_IsError = 1
SET #m_CatchEndless = #m_CatchEndless + 1
IF #m_CatchEndless > 1000
THROW 51000, '[upCreateRequestFromPattern]: Unable to get new RequestID', 1
END CATCH
This should work:
INSERT INTO Table1 (id, data_field)
SELECT (SELECT (MAX(id) + 1) FROM Table1), '[blob of data]';
Or this (substitute LIMIT for other platforms):
INSERT INTO Table1 (id, data_field)
SELECT TOP 1
MAX(id) + 1, '[blob of data]'
FROM
Table1
ORDER BY
[id] DESC;
Assume a table structure of MyTable(KEY, datafield1, datafield2...).
Often I want to either update an existing record, or insert a new record if it doesn't exist.
Essentially:
IF (key exists)
run update command
ELSE
run insert command
What's the best performing way to write this?
don't forget about transactions. Performance is good, but simple (IF EXISTS..) approach is very dangerous.
When multiple threads will try to perform Insert-or-update you can easily
get primary key violation.
Solutions provided by #Beau Crawford & #Esteban show general idea but error-prone.
To avoid deadlocks and PK violations you can use something like this:
begin tran
if exists (select * from table with (updlock,serializable) where key = #key)
begin
update table set ...
where key = #key
end
else
begin
insert into table (key, ...)
values (#key, ...)
end
commit tran
or
begin tran
update table with (serializable) set ...
where key = #key
if ##rowcount = 0
begin
insert into table (key, ...) values (#key,..)
end
commit tran
See my detailed answer to a very similar previous question
#Beau Crawford's is a good way in SQL 2005 and below, though if you're granting rep it should go to the first guy to SO it. The only problem is that for inserts it's still two IO operations.
MS Sql2008 introduces merge from the SQL:2003 standard:
merge tablename with(HOLDLOCK) as target
using (values ('new value', 'different value'))
as source (field1, field2)
on target.idfield = 7
when matched then
update
set field1 = source.field1,
field2 = source.field2,
...
when not matched then
insert ( idfield, field1, field2, ... )
values ( 7, source.field1, source.field2, ... )
Now it's really just one IO operation, but awful code :-(
Do an UPSERT:
UPDATE MyTable SET FieldA=#FieldA WHERE Key=#Key
IF ##ROWCOUNT = 0
INSERT INTO MyTable (FieldA) VALUES (#FieldA)
http://en.wikipedia.org/wiki/Upsert
Many people will suggest you use MERGE, but I caution you against it. By default, it doesn't protect you from concurrency and race conditions any more than multiple statements, and it introduces other dangers:
Use Caution with SQL Server's MERGE Statement
So, you want to use MERGE, eh?
Even with this "simpler" syntax available, I still prefer this approach (error handling omitted for brevity):
BEGIN TRANSACTION;
UPDATE dbo.table WITH (UPDLOCK, SERIALIZABLE)
SET ... WHERE PK = #PK;
IF ##ROWCOUNT = 0
BEGIN
INSERT dbo.table(PK, ...) SELECT #PK, ...;
END
COMMIT TRANSACTION;
Please stop using this UPSERT anti-pattern
A lot of folks will suggest this way:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
IF EXISTS (SELECT 1 FROM dbo.table WHERE PK = #PK)
BEGIN
UPDATE ...
END
ELSE
BEGIN
INSERT ...
END
COMMIT TRANSACTION;
But all this accomplishes is ensuring you may need to read the table twice to locate the row(s) to be updated. In the first sample, you will only ever need to locate the row(s) once. (In both cases, if no rows are found from the initial read, an insert occurs.)
Others will suggest this way:
BEGIN TRY
INSERT ...
END TRY
BEGIN CATCH
IF ERROR_NUMBER() = 2627
UPDATE ...
END CATCH
However, this is problematic if for no other reason than letting SQL Server catch exceptions that you could have prevented in the first place is much more expensive, except in the rare scenario where almost every insert fails. I prove as much here:
Checking for potential constraint violations before entering TRY/CATCH
Performance impact of different error handling techniques
IF EXISTS (SELECT * FROM [Table] WHERE ID = rowID)
UPDATE [Table] SET propertyOne = propOne, property2 . . .
ELSE
INSERT INTO [Table] (propOne, propTwo . . .)
Edit:
Alas, even to my own detriment, I must admit the solutions that do this without a select seem to be better since they accomplish the task with one less step.
If you want to UPSERT more than one record at a time you can use the ANSI SQL:2003 DML statement MERGE.
MERGE INTO table_name WITH (HOLDLOCK) USING table_name ON (condition)
WHEN MATCHED THEN UPDATE SET column1 = value1 [, column2 = value2 ...]
WHEN NOT MATCHED THEN INSERT (column1 [, column2 ...]) VALUES (value1 [, value2 ...])
Check out Mimicking MERGE Statement in SQL Server 2005.
Although its pretty late to comment on this I want to add a more complete example using MERGE.
Such Insert+Update statements are usually called "Upsert" statements and can be implemented using MERGE in SQL Server.
A very good example is given here:
http://weblogs.sqlteam.com/dang/archive/2009/01/31/UPSERT-Race-Condition-With-MERGE.aspx
The above explains locking and concurrency scenarios as well.
I will be quoting the same for reference:
ALTER PROCEDURE dbo.Merge_Foo2
#ID int
AS
SET NOCOUNT, XACT_ABORT ON;
MERGE dbo.Foo2 WITH (HOLDLOCK) AS f
USING (SELECT #ID AS ID) AS new_foo
ON f.ID = new_foo.ID
WHEN MATCHED THEN
UPDATE
SET f.UpdateSpid = ##SPID,
UpdateTime = SYSDATETIME()
WHEN NOT MATCHED THEN
INSERT
(
ID,
InsertSpid,
InsertTime
)
VALUES
(
new_foo.ID,
##SPID,
SYSDATETIME()
);
RETURN ##ERROR;
/*
CREATE TABLE ApplicationsDesSocietes (
id INT IDENTITY(0,1) NOT NULL,
applicationId INT NOT NULL,
societeId INT NOT NULL,
suppression BIT NULL,
CONSTRAINT PK_APPLICATIONSDESSOCIETES PRIMARY KEY (id)
)
GO
--*/
DECLARE #applicationId INT = 81, #societeId INT = 43, #suppression BIT = 0
MERGE dbo.ApplicationsDesSocietes WITH (HOLDLOCK) AS target
--set the SOURCE table one row
USING (VALUES (#applicationId, #societeId, #suppression))
AS source (applicationId, societeId, suppression)
--here goes the ON join condition
ON target.applicationId = source.applicationId and target.societeId = source.societeId
WHEN MATCHED THEN
UPDATE
--place your list of SET here
SET target.suppression = source.suppression
WHEN NOT MATCHED THEN
--insert a new line with the SOURCE table one row
INSERT (applicationId, societeId, suppression)
VALUES (source.applicationId, source.societeId, source.suppression);
GO
Replace table and field names by whatever you need.
Take care of the using ON condition.
Then set the appropriate value (and type) for the variables on the DECLARE line.
Cheers.
That depends on the usage pattern. One has to look at the usage big picture without getting lost in the details. For example, if the usage pattern is 99% updates after the record has been created, then the 'UPSERT' is the best solution.
After the first insert (hit), it will be all single statement updates, no ifs or buts. The 'where' condition on the insert is necessary otherwise it will insert duplicates, and you don't want to deal with locking.
UPDATE <tableName> SET <field>=#field WHERE key=#key;
IF ##ROWCOUNT = 0
BEGIN
INSERT INTO <tableName> (field)
SELECT #field
WHERE NOT EXISTS (select * from tableName where key = #key);
END
You can use MERGE Statement, This statement is used to insert data if not exist or update if does exist.
MERGE INTO Employee AS e
using EmployeeUpdate AS eu
ON e.EmployeeID = eu.EmployeeID`
If going the UPDATE if-no-rows-updated then INSERT route, consider doing the INSERT first to prevent a race condition (assuming no intervening DELETE)
INSERT INTO MyTable (Key, FieldA)
SELECT #Key, #FieldA
WHERE NOT EXISTS
(
SELECT *
FROM MyTable
WHERE Key = #Key
)
IF ##ROWCOUNT = 0
BEGIN
UPDATE MyTable
SET FieldA=#FieldA
WHERE Key=#Key
IF ##ROWCOUNT = 0
... record was deleted, consider looping to re-run the INSERT, or RAISERROR ...
END
Apart from avoiding a race condition, if in most cases the record will already exist then this will cause the INSERT to fail, wasting CPU.
Using MERGE probably preferable for SQL2008 onwards.
MS SQL Server 2008 introduces the MERGE statement, which I believe is part of the SQL:2003 standard. As many have shown it is not a big deal to handle one row cases, but when dealing with large datasets, one needs a cursor, with all the performance problems that come along. The MERGE statement will be much welcomed addition when dealing with large datasets.
Before everyone jumps to HOLDLOCK-s out of fear from these nafarious users running your sprocs directly :-) let me point out that you have to guarantee uniqueness of new PK-s by design (identity keys, sequence generators in Oracle, unique indexes for external ID-s, queries covered by indexes). That's the alpha and omega of the issue. If you don't have that, no HOLDLOCK-s of the universe are going to save you and if you do have that then you don't need anything beyond UPDLOCK on the first select (or to use update first).
Sprocs normally run under very controlled conditions and with the assumption of a trusted caller (mid tier). Meaning that if a simple upsert pattern (update+insert or merge) ever sees duplicate PK that means a bug in your mid-tier or table design and it's good that SQL will yell a fault in such case and reject the record. Placing a HOLDLOCK in this case equals eating exceptions and taking in potentially faulty data, besides reducing your perf.
Having said that, Using MERGE, or UPDATE then INSERT is easier on your server and less error prone since you don't have to remember to add (UPDLOCK) to first select. Also, if you are doing inserts/updates in small batches you need to know your data in order to decide whether a transaction is appropriate or not. It it's just a collection of unrelated records then additional "enveloping" transaction will be detrimental.
Does the race conditions really matter if you first try an update followed by an insert?
Lets say you have two threads that want to set a value for key key:
Thread 1: value = 1
Thread 2: value = 2
Example race condition scenario
key is not defined
Thread 1 fails with update
Thread 2 fails with update
Exactly one of thread 1 or thread 2 succeeds with insert. E.g. thread 1
The other thread fails with insert (with error duplicate key) - thread 2.
Result: The "first" of the two treads to insert, decides value.
Wanted result: The last of the 2 threads to write data (update or insert) should decide value
But; in a multithreaded environment, the OS scheduler decides on the order of the thread execution - in the above scenario, where we have this race condition, it was the OS that decided on the sequence of execution. Ie: It is wrong to say that "thread 1" or "thread 2" was "first" from a system viewpoint.
When the time of execution is so close for thread 1 and thread 2, the outcome of the race condition doesn't matter. The only requirement should be that one of the threads should define the resulting value.
For the implementation: If update followed by insert results in error "duplicate key", this should be treated as success.
Also, one should of course never assume that value in the database is the same as the value you wrote last.
I had tried below solution and it works for me, when concurrent request for insert statement occurs.
begin tran
if exists (select * from table with (updlock,serializable) where key = #key)
begin
update table set ...
where key = #key
end
else
begin
insert table (key, ...)
values (#key, ...)
end
commit tran
You can use this query. Work in all SQL Server editions. It's simple, and clear. But you need use 2 queries. You can use if you can't use MERGE
BEGIN TRAN
UPDATE table
SET Id = #ID, Description = #Description
WHERE Id = #Id
INSERT INTO table(Id, Description)
SELECT #Id, #Description
WHERE NOT EXISTS (SELECT NULL FROM table WHERE Id = #Id)
COMMIT TRAN
NOTE: Please explain answer negatives
Assuming that you want to insert/update single row, most optimal approach is to use SQL Server's REPEATABLE READ transaction isolation level:
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
BEGIN TRANSACTION
IF (EXISTS (SELECT * FROM myTable WHERE key=#key)
UPDATE myTable SET ...
WHERE key=#key
ELSE
INSERT INTO myTable (key, ...)
VALUES (#key, ...)
COMMIT TRANSACTION
This isolation level will prevent/block subsequent repeatable read transactions from accessing same row (WHERE key=#key) while currently running transaction is open.
On the other hand, operations on another row won't be blocked (WHERE key=#key2).
You can use:
INSERT INTO tableName (...) VALUES (...)
ON DUPLICATE KEY
UPDATE ...
Using this, if there is already an entry for the particular key, then it will UPDATE, else, it will INSERT.
In SQL Server 2008 you can use the MERGE statement
If you use ADO.NET, the DataAdapter handles this.
If you want to handle it yourself, this is the way:
Make sure there is a primary key constraint on your key column.
Then you:
Do the update
If the update fails because a record with the key already exists, do the insert. If the update does not fail, you are finished.
You can also do it the other way round, i.e. do the insert first, and do the update if the insert fails. Normally the first way is better, because updates are done more often than inserts.
Doing an if exists ... else ... involves doing two requests minimum (one to check, one to take action). The following approach requires only one where the record exists, two if an insert is required:
DECLARE #RowExists bit
SET #RowExists = 0
UPDATE MyTable SET DataField1 = 'xxx', #RowExists = 1 WHERE Key = 123
IF #RowExists = 0
INSERT INTO MyTable (Key, DataField1) VALUES (123, 'xxx')
I usually do what several of the other posters have said with regard to checking for it existing first and then doing whatever the correct path is. One thing you should remember when doing this is that the execution plan cached by sql could be nonoptimal for one path or the other. I believe the best way to do this is to call two different stored procedures.
FirstSP:
If Exists
Call SecondSP (UpdateProc)
Else
Call ThirdSP (InsertProc)
Now, I don't follow my own advice very often, so take it with a grain of salt.
Do a select, if you get a result, update it, if not, create it.