I have created my first trigger. Please see the code section for the trigger below.
The triggers and the results are as expected, except for one thing.
So when I run the code below it will not insert the values into my table so the number of records remains unchanged.
insert into MatlabSearchPath(directory, userName)
values('madeup', 'default')
In the messages window though I get two lines. I don't understand why I see two lines and in particular 1 row affected - the number of records in my table hasn't changed?
(0 row(s) affected)
(1 row(s) affected)
Trigger
create trigger trDefaultPathInsert on DVLP_QES.dbo.MatlabSearchPath
instead of insert
as
begin
declare #defCount int
declare #retVal int
select #defCount = count(userName) from inserted where userName = 'Default'
if (#defCount > 0)
begin
select #retVal = count(HostName) from DVLP_QES.dbo.UserHostName where HostName = HOST_NAME()
if (#retVal > 0)
begin
insert into MatlabSearchPath select * from inserted
end
else
begin
insert into MatlabSearchPath select * from inserted where inserted.userName <> 'Default'
end
end
end
Update
I should mention that there a 3 triggers on this table, one is the trigger above the other one is a delete & the last one is an update
Your trigger does the following:
Counts records you are trying to insert, where userName equals 'Default'
In your case, count is 1.
Pay attention to your collation - if it's case sensitive, you are going to skip that whole branch of code.
If you enter the if branch, next thing trigger checks is if there are rows in UserHostName table where HostName equals host name of your client; pay attention that you don't think it should be host name of your server or something like that
If you enter the TRUE-branch, it should insert everything to the table; however, if not, it shouldn't insert anything. Of course, except if the collation is case sensitive, then revert the logic.
I I were you, I would add PRINT statements into trigger, just to make sure how does it execute.
create trigger trDefaultPathInsert on DVLP_QES.dbo.MatlabSearchPath
instead of insert
as
begin
declare #defCount int
declare #retVal int
select #defCount = count(userName) from inserted where userName = 'Default'
PRINT '#defCount'
PRINT #defCount
if (#defCount > 0)
begin
select #retVal = count(HostName) from DVLP_QES.dbo.UserHostName where HostName = HOST_NAME()
PRINT '#retVal'
PRINT #retVal
if (#retVal > 0)
begin
PRINT 'TRUE-BRANCH'
insert into MatlabSearchPath select * from inserted
end
else
begin
PRINT 'FALSE-BRANCH'
insert into MatlabSearchPath select * from inserted where inserted.userName <> 'Default'
end
end
EDIT
It seems that the message about rows affected can't be controlled inside the trigger. Even the standard SET NOCOUNT ON on the trigger beginning won't stop it from showing. This gave me notion that the message is a result of the trigger being successfully finished by calling it with X rows, where X will eventually be in the X row(s) affected message.
This SO question furtherly confirms the problem.
The situation here if the first message indicating cero is because the instead of trigger is uses to ignore the insert you sent and do instead whats in the trigger
You can debug your code with management studio
Related
There is a table with IDU (PK) and stat columns. If first bit of stat is 1 I need to set it to 0 and run some stored procedure in this case only, otherwise I do nothing.
Here is the simple query for this
DECLARE #s INT
-- get the current value of status before update
SET #s = (SELECT stat FROM myTable
WHERE IDU = 999999999)
-- check it first bit is 1
IF (#s & 1) = 1
BEGIN
-- first bit is 1, set it to 0
UPDATE myTable
SET status = Stat & ~1
WHERE IDU = 999999999
-- first bit is one, in this case we run our SP
EXEC SOME_STORED_PROCEDURE
END
But I'm not sure that this query is optimal. I heard about OUTPUT parameter for UPDATE query but I found how to get inserted value. Is there a way to get a value that was before insert?
Yes, OUTPUT clause allows you to get the previous value before the update. You need to look at deleted and inserted tables.
DELETED
Is a column prefix that specifies the value deleted by the
update or delete operation. Columns prefixed with DELETED reflect the
value before the UPDATE, DELETE, or MERGE statement is completed.
INSERTED
Is a column prefix that specifies the value added by the insert or
update operation. Columns prefixed with INSERTED reflect the value
after the UPDATE, INSERT, or MERGE statement is completed but before
triggers are executed.
-- Clear the first bit without checking what it was
DECLARE #Results TABLE (OldStat int, NewStat int);
UPDATE myTable
SET Stat = Stat & ~1
WHERE IDU = 999999999
OUTPUT
deleted.Stat AS OldStat
,inserted.Stat AS NewStat
INTO #Results
;
-- Copy data from #Results table into variables for comparison
-- Assumes that IDU is a primary key and #Results can have only one row
DECLARE #OldStat int;
DECLARE #NewStat int;
SELECT #OldStat = OldStat, #NewStat = NewStat
FROM #Results;
IF #OldStat <> #NewStat
BEGIN
EXEC SOME_STORED_PROCEDURE;
END;
Regardless of optimal, this query is not 100% safe. This is because between SET #s =... and UPDATE myTable there is no guarantee the value of stat has not been changed. If this code runs multiple times it is possible to screw up if two cases execute deadly close for the same IDU. The first thread will do ok but the second one will not, since the first would change the stat after the second read it and before update it. A select statement does not lock beyond its own execution time even on SERIALIZABLE isolation.
To be safe, you need to lock the record BEFORE read it, and to do that you need an update statement, even fake:
DECLARE #s INT
BEGIN TRANSACTION
UPDATE myTable SET stat = stat WHERE IDU = 999999999 --now you row lock your row, make sure no other thread can move along
-- get the current value of status before update
SET #s = (SELECT stat FROM myTable
WHERE IDU = 999999999)
-- check it first bit is 1
IF (#s & 1) = 1
BEGIN
-- first bit is 1, set it to 0
UPDATE myTable
SET status = Stat & ~1
WHERE IDU = 999999999
-- first bit is one, in this case we run our SP
-- COMMIT TRANSACTION here? depends on what SOME_STORED_PROCEDURE does
EXEC SOME_STORED_PROCEDURE
COMMIT TRANSACTION --i believe here you release the row lock
I am not sure what you mean by "Is there a way to get a value that was before insert" because you only update and the only data, stat, you had already read from the old record regardless if you update or not.
You could do this with an INSTEAD OF UPDATE Trigger.
I need to write a trigger which prevents from inserting more than one record at the same time and also checks if the place is already in the database. Code compiles but it doesn't work as it should - it displays error message even if I try to add a non-existing address
Here's my code:
CREATE TRIGGER address_duplicate ON place
AFTER INSERT
AS
BEGIN
DECLARE #counter INT
SELECT #counter=COUNT(*) FROM place WHERE street IN (SELECT street FROM inserted) AND number IN(SELECT number FROM inserted)
AND city IN(SELECT city FROM inserted) AND postcode IN(SELECT postcode FROM inserted)
IF #counter>0
BEGIN
RAISERROR('This record is already in the database',1,1)
ROLLBACK
END
IF ##ROWCOUNT>1
BEGIN
RAISERROR('You can add only one record at the same time',1,2)
ROLLBACK
END
END
GO
Your logic of identifying duplicate place is not correct.
Try something like this:
select #counter= count(*) from place p join inserted n
where p.address=n.address and p.city=n.city and p.postcode=n.postcode
and p.number=n.number;
Also want you to know, using triggers to avoid duplicate can be very expensive.
Personally, I'd use a unique constraint and probably use TRY-CATCH when inserting, but if you really want to do it in a trigger, try this out:
CREATE TRIGGER address_duplicate ON place
INSTEAD OF INSERT
AS
BEGIN
DECLARE #newValue TABLE (ID INT);
IF ##ROWCOUNT > 1
BEGIN
RAISERROR('Insert cancelled. You can add only one record at the same time.',1,2);
END
ELSE
BEGIN
INSERT INTO place
OUTPUT inserted.ID INTO #newValue
SELECT *
FROM inserted
EXCEPT
SELECT *
FROM place
IF (SELECT 1 FROM #newValue) IS NULL
RAISERROR('Insert cancelled. This record is already in the database',1,1)
END
END
I need to create a unique identifier for clients, I'm implementing this using a 9 digit Luhn (9th digit being the checkdigit), so I can validate its authenticity. The numbers I want to generate are random, so I create an 8 digit number and work out the check digit to go with it, that's all great.
My problem is that I need to check that it doesn't already exist in my client table. I have added an index to ensure duplicate values can't be inserted, but am looking for a steer on how to lock the table involved within the transaction I've created to ensure no concurrency issues arise, i.e. can't attempt to insert a duplicate.
Any suggestions on approach or recommendations welcome, I know this is probably a noddy question.
The short answer is that you don't have direct control over which type of lock is used and when. You do have the ability, however, to place multiple statements within an explicitly defined transaction.
At the end of the transaction, you can check to see if there were errors in processing or if your data is in an invalid state. Then you would commit if everything is fine or rollback if not. But, beware of leaving transactions open! If you don't commit or rollback your transaction, the affected tables will remain locked and all transactions against those tables will be blocked until you either commit or rollback the transaction.
Here is an example that will hopefully be of help:
DECLARE #Table TABLE
(
ID INT IDENTITY PRIMARY KEY
,Luhn INT
,UNIQUE(Luhn)
)
DECLARE #MaxTries INT = 10;
DECLARE #Try INT = 1;
DECLARE #Luhn INT;
DECLARE #IsLuhnAvailable BIT = 0;
BEGIN TRAN;
WHILE #IsLuhnAvailable = 0 AND #Try <= #MaxTries
BEGIN
SET #Luhn = CHECKSUM(NEWID()); --dbo.GenerateLuhn()
SET #IsLuhnAvailable = CASE WHEN EXISTS ( SELECT 1 FROM #Table WHERE Luhn = #Luhn ) THEN 0 ELSE 1 END;
SET #Try +=1;
END
IF #IsLuhnAvailable = 0
BEGIN
PRINT 'Luhn could not be generated.'
ROLLBACK;
END
ELSE
BEGIN
INSERT INTO #Table
(
Luhn
)
VALUES
(
#Luhn
)
PRINT 'New Luhn was generated: ' + CAST(#Luhn AS VARCHAR)
COMMIT;
END
SELECT * FROM #Table
Here is a trigger
CREATE TRIGGER [dbo].[CheckApplyId]
ON [dbo].[AppliedStudent_event] INSTEAD OF INSERT
AS
DECLARE #studentId INT
DECLARE #compReq_Id INT
BEGIN
SELECT #studentId = studentId
FROM INSERTED
SELECT #compReq_Id = compReq_Id
FROM INSERTED
IF EXISTS(SELECT StudentId,
compreq_id
FROM AppliedStudent_event
WHERE StudentId = #studentId
AND compreq_id = #compReq_Id)
BEGIN
ROLLBACK
PRINT 'User Already Applied'
END
END
When in insert a data into a table using command:
INSERT INTO AppliedStudent_event (StudentId, compreq_id)
VALUES (3026, 1)
Message is:
(1 row(s) affected)
But when I execute a sql command no data is inserted in the table.
Can you please tell why are you using trigger because you use only assign the variable #studentId and #compReq_Id from inserted table.
That's a broken trigger because inserted can contain multiple (or no) rows - so a statement like SELECT #ScalarVariable = column from inserted is always wrong.
And it's unnecessary since you can just place a UNIQUE constraint on the StudentId and compreq_id columns:
ALTER TABLE AppliedStudent_event
ADD CONSTRAINT UQ_Student_Events
UNIQUE (StudentId,compreq_id)
And it's further broken because you've specified it as an instead of trigger - that says that your code is going to be responsible for the actual insert - but your code doesn't actually do that. That's why no data ends up in the table.
If you insist on doing it as a trigger, it's actually tricky to get everything correct (that's why I'd really recommend the UNIQUE constraint). It'll end up being something like this:
CREATE TRIGGER [dbo].[CheckApplyId]
ON [dbo].[AppliedStudent_event] INSTEAD OF INSERT
AS
IF EXISTS(select
StudentId,compreq_id,COUNT(*)
from inserted
group by StudentId,compreq_id
HAVING COUNT(*) > 1)
OR EXISTS (select *
from inserted i
inner join
AppliedStudent_event e
on
i.StudentId = e.StudentId and
i.compreq_id = e.compreq_id)
BEGIN
ROLLBACK
PRINT 'User Already Applied'
END
ELSE
BEGIN
INSERT INTO AppliedStudent_event(StudentId,compreq_id /* Other columns? */)
SELECT StudentId,compreq_id /* And again, other columns */
FROM inserted
END
I've got a table where I need to auto-assign an ID 99% of the time (the other 1% rules out using an identity column it seems). So I've got a stored procedure to get next ID along the following lines:
select #nextid = lastid+1 from last_auto_id
check next available id in the table...
update last_auto_id set lastid = #nextid
Where the check has to check if users have manually used the IDs and find the next unused ID.
It works fine when I call it serially, returning 1, 2, 3 ... What I need to do is provide some locking where multiple processes call this at the same time. Ideally, I just need it to exclusively lock the last_auto_id table around this code so that a second call must wait for the first to update the table before it can run it's select.
In Postgres, I can do something like 'LOCK TABLE last_auto_id;' to explicitly lock the table. Any ideas how to accomplish it in SQL Server?
Thanks in advance!
Following update increments your lastid by one and assigns this value to your local variable in a single transaction.
Edit
thanks to Dave and Mitch for pointing out isolation level problems with the original solution.
UPDATE last_auto_id WITH (READCOMMITTEDLOCK)
SET #nextid = lastid = lastid + 1
You guys have between you answered my question. I'm putting in my own reply to collate the working solution I've got into one post. The key seems to have been the transaction approach, with locking hints on the last_auto_id table. Setting the transaction isolation to serializable seemed to create deadlock problems.
Here's what I've got (edited to show the full code so hopefully I can get some further answers...):
DECLARE #Pointer AS INT
BEGIN TRANSACTION
-- Check what the next ID to use should be
SELECT #NextId = LastId + 1 FROM Last_Auto_Id WITH (TABLOCKX) WHERE Name = 'CustomerNo'
-- Now check if this next ID already exists in the database
IF EXISTS (SELECT CustomerNo FROM Customer
WHERE ISNUMERIC(CustomerNo) = 1 AND CustomerNo = #NextId)
BEGIN
-- The next ID already exists - we need to find the next lowest free ID
CREATE TABLE #idtbl ( IdNo int )
-- Into temp table, grab all numeric IDs higher than the current next ID
INSERT INTO #idtbl
SELECT CAST(CustomerNo AS INT) FROM Customer
WHERE ISNUMERIC(CustomerNo) = 1 AND CustomerNo >= #NextId
ORDER BY CAST(CustomerNo AS INT)
-- Join the table with itself, based on the right hand side of the join
-- being equal to the ID on the left hand side + 1. We're looking for
-- the lowest record where the right hand side is NULL (i.e. the ID is
-- unused)
SELECT #Pointer = MIN( t1.IdNo ) + 1 FROM #idtbl t1
LEFT OUTER JOIN #idtbl t2 ON t1.IdNo + 1 = t2.IdNo
WHERE t2.IdNo IS NULL
END
UPDATE Last_Auto_Id SET LastId = #NextId WHERE Name = 'CustomerNo'
COMMIT TRANSACTION
SELECT #NextId
This takes out an exclusive table lock at the start of the transaction, which then successfully queues up any further requests until after this request has updated the table and committed it's transaction.
I've written a bit of C code to hammer it with concurrent requests from half a dozen sessions and it's working perfectly.
However, I do have one worry which is the term locking 'hints' - does anyone know if SQLServer treats this as a definite instruction or just a hint (i.e. maybe it won't always obey it??)
How is this solution? No TABLE LOCK is required and works perfectly!!!
DECLARE #NextId INT
UPDATE Last_Auto_Id
SET #NextId = LastId = LastId + 1
WHERE Name = 'CustomerNo'
SELECT #NextId
Update statement always uses a lock to protect its update.
You might wanna consider deadlocks. This usually happens when multiple users use the stored procedure simultaneously. In order to avoid deadlock and make sure every query from the user will succeed you will need to do some handling during update failures and to do this you will need a try catch. This works on Sql Server 2005,2008 only.
DECLARE #Tries tinyint
SET #Tries = 1
WHILE #Tries <= 3
BEGIN
BEGIN TRANSACTION
BEGIN TRY
-- this line updates the last_auto_id
update last_auto_id set lastid = lastid+1
COMMIT
BREAK
END TRY
BEGIN CATCH
SELECT ERROR_NUMBER() AS ErrorNumber, ERROR_MESSAGE() as ErrorMessage
ROLLBACK
SET #Tries = #Tries + 1
CONTINUE
END CATCH
END
I prefer doing this using an identity field in a second table. If you make lastid identity then all you have to do is insert a row in that table and select #scope_identity to get your new value and you still have the concurrency safety of identity even though the id field in your main table is not identity.