Related
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 want to insert a row and return the identity column value from a procedure! For this I dont want to use any advanced things like ##identity or SCOPE_IDENTITY etc as I don't have a clear picture about how they work.
So I am planning to do this
lock a table
do the insert
return the max of table
I am familiar with oracle lock
lock table tab1 in exclusive mode;
INSERT INTO tab1.....;
select max(id) into retVal from tab1;
Questions:
Is there anything wrong with my approach?
How can i achieve this in SQL SERVER?
which version of sql server it is?
If you are running sql server 2008+ (r2 i thing)
The safest way is to use the output statement of an insert.
Like this :
insert into Customer (Name, Address, Phone)
output inserted.CustomerId into #CustomerId
values (......)
You'll be creating more problems than solving by forcing locks on your database/tables. Let the Database Management System (DBMS) manage the database.
All you need to do is:
BEGIN TRANSACTION --added after original answer
INSERT INTO tab1....;
SELECT ##identity AS [last_id] INTO retVal;
END TRANSACTION --added after original answer
I trust you know that you can only SELECT...INTO retVal so long as the retVal table doesn't already exist.
Typically, I test an update by running a query using the where statement and then after verifying that it does what I think I want it to copying the where clause to the update statement and executing it. But is there any way of getting the statement to return what the update did besides the '4 rows updated'
Sure, take a look at the output clause of T-SQL
http://msdn.microsoft.com/en-us/library/ms177564.aspx
You could load your records into a temp table/variable in SQL Server:
DECLARE #Temp TABLE(ID INT)
INSERT INTO #Temp (ID)
SELECT ID
FROM Customer
WHERE AcctBalance > 5000
--inspect as needed
UPDATE Customer
SET AcctBalance = 0
WHERE ID IN (SELECT ID FROM #Temp)
That depend in the server, library that you use, in php, pdo exec return number of row effected by delete or update cluase
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've written a stored proc that will do an update if a record exists, otherwise it will do an insert. It looks something like this:
update myTable set Col1=#col1, Col2=#col2 where ID=#ID
if ##rowcount = 0
insert into myTable (Col1, Col2) values (#col1, #col2)
My logic behind writing it in this way is that the update will perform an implicit select using the where clause and if that returns 0 then the insert will take place.
The alternative to doing it this way would be to do a select and then based on the number of rows returned either do an update or insert. This I considered inefficient because if you are to do an update it will cause 2 selects (the first explicit select call and the second implicit in the where of the update). If the proc were to do an insert then there'd be no difference in efficiency.
Is my logic sound here?
Is this how you would combine an insert and update into a stored proc?
Your assumption is right, this is the optimal way to do it and it's called upsert/merge.
Importance of UPSERT - from sqlservercentral.com:
For every update in the case mentioned above we are removing one
additional read from the table if we
use the UPSERT instead of EXISTS.
Unfortunately for an Insert, both the
UPSERT and IF EXISTS methods use the
same number of reads on the table.
Therefore the check for existence
should only be done when there is a
very valid reason to justify the
additional I/O. The optimized way to
do things is to make sure that you
have little reads as possible on the
DB.
The best strategy is to attempt the
update. If no rows are affected by the
update then insert. In most
circumstances, the row will already
exist and only one I/O will be
required.
Edit:
Please check out this answer and the linked blog post to learn about the problems with this pattern and how to make it work safe.
Please read the post on my blog for a good, safe pattern you can use. There are a lot of considerations, and the accepted answer on this question is far from safe.
For a quick answer try the following pattern. It will work fine on SQL 2000 and above. SQL 2005 gives you error handling which opens up other options and SQL 2008 gives you a MERGE command.
begin tran
update t with (serializable)
set hitCount = hitCount + 1
where pk = #id
if ##rowcount = 0
begin
insert t (pk, hitCount)
values (#id,1)
end
commit tran
If to be used with SQL Server 2000/2005 the original code needs to be enclosed in transaction to make sure that data remain consistent in concurrent scenario.
BEGIN TRANSACTION Upsert
update myTable set Col1=#col1, Col2=#col2 where ID=#ID
if ##rowcount = 0
insert into myTable (Col1, Col2) values (#col1, #col2)
COMMIT TRANSACTION Upsert
This will incur additional performance cost, but will ensure data integrity.
Add, as already suggested, MERGE should be used where available.
MERGE is one of the new features in SQL Server 2008, by the way.
You not only need to run it in transaction, it also needs high isolation level. I fact default isolation level is Read Commited and this code need Serializable.
SET transaction isolation level SERIALIZABLE
BEGIN TRANSACTION Upsert
UPDATE myTable set Col1=#col1, Col2=#col2 where ID=#ID
if ##rowcount = 0
begin
INSERT into myTable (ID, Col1, Col2) values (#ID #col1, #col2)
end
COMMIT TRANSACTION Upsert
Maybe adding also the ##error check and rollback could be good idea.
If you are not doing a merge in SQL 2008 you must change it to:
if ##rowcount = 0 and ##error=0
otherwise if the update fails for some reason then it will try and to an insert afterwards because the rowcount on a failed statement is 0
Big fan of the UPSERT, really cuts down on the code to manage. Here is another way I do it: One of the input parameters is ID, if the ID is NULL or 0, you know it's an INSERT, otherwise it's an update. Assumes the application knows if there is an ID, so wont work in all situations, but will cut the executes in half if you do.
Modified Dima Malenko post:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION UPSERT
UPDATE MYTABLE
SET COL1 = #col1,
COL2 = #col2
WHERE ID = #ID
IF ##rowcount = 0
BEGIN
INSERT INTO MYTABLE
(ID,
COL1,
COL2)
VALUES (#ID,
#col1,
#col2)
END
IF ##Error > 0
BEGIN
INSERT INTO MYERRORTABLE
(ID,
COL1,
COL2)
VALUES (#ID,
#col1,
#col2)
END
COMMIT TRANSACTION UPSERT
You can trap the error and send the record to a failed insert table.
I needed to do this because we are taking whatever data is send via WSDL and if possible fixing it internally.
Your logic seems sound, but you might want to consider adding some code to prevent the insert if you had passed in a specific primary key.
Otherwise, if you're always doing an insert if the update didn't affect any records, what happens when someone deletes the record before you "UPSERT" runs? Now the record you were trying to update doesn't exist, so it'll create a record instead. That probably isn't the behavior you were looking for.