Recently, I needed to write a stored procedure to insert only one record when the first user come and ignore for others. I think the IF NOT EXISTS INSERT will not work for me. Also, some people saying online that MERGE adds race condition. Any quick way to achieve this? This is my code for now.
IF NOT EXISTS (SELECT ......)
INSERT
You might add another table to use as the lock mechanism.
Let's say your table's name is a, and the name of the table which has the locked value is check_a :
create table a (name varchar(10))
create table check_a (name varchar(10))
Insert only one record to the lock table:
insert into check_a values ('lock')
go
Then create a stored procedure which checks if there is a value in the main table. If there is no record, we might lock the only value in the table check_a and insert our value into the table a.
create proc insert_if_first
as
begin
set nocount on
if not exists (select name from a)
begin
declare #name varchar(10)
begin tran
select #name = name from check_a with (updlock)
if not exists (select name from a)
begin
insert into a values ('some value')
end
commit
end
end
go
First selection from the table a to check there is no record is for using system resources as low as we can. If there is a record in the table a, we can skip opening transaction and skip locking the row.
Second check is to make sure that while we are waiting to obtain the lock, no one inserted a row to the table a.
This way, only the first user which can lock check_a will be able to insert a value to the table a.
I'm guessing that you mean you want users to make a stored procedure that makes sure only one user can run the procedure. Then you need to use isolation levels. There are different Isolation levels, so you need to decide which one you need.
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
You can read what they do here:
https://msdn.microsoft.com/en-us/library/ms173763.aspx
Related
I am trying to accomplish the following 3 simple tasks as a transaction (i.e. I need to lock old_table and new_table until the process completes).
Create a new table (new_table)
Add a trigger to old_table, which queues updates to new_table.
Select all the data from old_table and return it.
Note that I want these handled in a single transaction. I cannot allow inserts into old_table (and therefore triggered inserts into new_table) in between the trigger creation and the select on old_table.
My current closest attempt is this, but truthfully I feel that I am very far off from accomplishing my goal with this code. I have added the code just for reference of what I am trying, but I am mostly interested in non-specific answers that layout how to accomplish the above three comands in a transaction.
DROP PROCEDURE IF EXISTS dbo.BuildAll;
CREATE PROCEDURE dbo.BuildAll
AS
BEGIN
BEGIN TRANSACTION
DECLARE #TriggerCode VARCHAR(MAX)
CREATE TABLE dbo.new_table
(
status nvarchar(5),
type char(1),
col1 nvarchar(50),
col2 smallint
)
SELECT #TriggerCode = 'CREATE TRIGGER myTrigger
ON dbo.old_table FOR INSERT
AS
DECLARE #col1_new nvarchar(50)
DECLARE #col2_new smallint
SELECT #col1_new = col1 FROM inserted
SELECT #col2_new = col2 FROM inserted
IF #col1_new IS NOT NULL
BEGIN
INSERT INTO new_table (status, type, col1, col2)
SELECt "Q", "A", #col1, #col2 FROM inserted
END'
EXEC(#TriggerCode)
SELECT * FROM old_table
COMMIT
END
Going to suggest this an a possible solution you can try. This doesn't address the correctness of your actual trigger, you have two separate questions here really.
You don't need to encapsulate this entire process in a transaction.
Create your new table.
Create your trigger on old table, but disabled.
set transaction isolation level serializable
begin tran
go
create trigger <Name> on <Table> etc
go
disable trigger <Name> on <Table>
go
commit
Now in a transaction you can lock the old table against other activity while you work
begin tran
update oldtable with(tablockx) set column=column where id=0 /* block other processes from updating table, id=0 row doesn't exist */
query your data and process as required
enable trigger <Name> on <Table>
commit
This trigger code of yours is kinda odd .... you have a trigger on all three operations - yet it appears as if you're never using the values you fetch from the deleted pseudo table, and if the value from the inserted table is NULL, you're not doing anything inside your trigger - so you can really spare yourself the DELETE case - that'll never do anything....
Also, as mentioned in my comment - you Inserted pseudo table can easily contain multiple rows - but you're selecting from it as if you only ever expect it to contain a single row.
You should really rewrite your trigger code to handle the case of multiple rows in Inserted and make the whole thing properly set-based - something like this:
CREATE TRIGGER myTrigger
ON dbo.old_table
FOR INSERT, UPDATE
AS
INSERT INTO new_table (status, type, col1, col2)
SELECT 'Q', 'A', i.col1, i.col2
FROM Inserted i
Whether you need this on the UPDATE case at all - I cannot tell, you need to decide this. But basically: just select from the Inserted table, take the Col1 and Col2 values, and add the constant values 'Q' and 'A' to your insert to handle multiple rows properly. That should do it.
I have a database that needs from time to time an update.
It may also happens that there are new data while the update runs.
In MySQL there is a option
INSERT INTO IGNORE
I can't find something like this in T-SQL.
No Problem to update ID 1-4 but then there is a new record for ID 5.
The UPDATE query don't work here.
And when I try to INSERT all data again I get a DUPLICATE KEY error.
Additional Infos:
I've forgotten to say that my data come from external sources. I call an API to get data from it. From there I have to insert these data into my database.
I have to admit that I don't understand MERGE. So my solution for now is to use TRUNCATE first and then insert all data again.
Not the best solution but MERGE works, so far I understand it, with two tables. But I have only one table. And to create a table temporarly to use MERGE and later drop that table is in my eyes a bit to much for my little table with 200 records in it.
You can use MERGE keyword. Basically, you need to specify the column(s) on which to join the source of data with target table, and depending on whether it is matching (existing record) or not matching (new record), you run an UPDATE or INSERT.
Reference: http://msdn.microsoft.com/en-us/library/bb510625.aspx
Is a stored procedure an option?
CREATE PROCEDURE dbo.Testing (#ID int, #Field1 varchar(20))
AS
BEGIN
UPDATE tblTesting
SET Field1 = #Field1
WHERE ID = #ID
IF ##ROWCOUNT = 0
INSERT INTO tblTesting (ID, Field1) SELECT #ID, #Field1
END
Is there a way to insert records into TABLE B if there is an update in TABLE A?
I don't want to use triggers.
The answer is we can use the OUTPUT clause of instead of triggers:
USE AdventureWorks2012;
GO
IF OBJECT_ID('dbo.vw_ScrapReason','V') IS NOT NULL
DROP VIEW dbo.vw_ScrapReason;
GO
CREATE VIEW dbo.vw_ScrapReason
AS (SELECT ScrapReasonID, Name, ModifiedDate
FROM Production.ScrapReason);
GO
CREATE TRIGGER dbo.io_ScrapReason
ON dbo.vw_ScrapReason
INSTEAD OF INSERT
AS
BEGIN
--ScrapReasonID is not specified in the list of columns to be inserted
--because it is an IDENTITY column.
INSERT INTO Production.ScrapReason (Name, ModifiedDate)
OUTPUT INSERTED.ScrapReasonID, INSERTED.Name,
INSERTED.ModifiedDate
SELECT Name, getdate()
FROM inserted;
END
GO
INSERT vw_ScrapReason (ScrapReasonID, Name, ModifiedDate)
VALUES (99, N'My scrap reason','20030404');
GO
The mechanism for doing this is called triggers.
Saying that you want to do this but don't want to use triggers is like saying you want to see the Eiffel Tower, but you don't want to go to France.
You could, I suppose, write a stored procedure that does all the logic that would have been in the trigger, if you can ensure that all data updates will be via that stored procedure.
If you don't want to use triggers, then you would have three options.
The first would be to wrap all inserts/updates/deletes in stored procedures. Then use only these stored procedures for data modification. This is actually the approach that I generally take.
Another would be to have a process that runs periodically looking for changes to the data. This is actually hard to do for updates. It is pretty easy to do for inserts because you can add an column with a default creation date, so you can readily find what has recently been added.
The third way is to use SQL Server Change Tracking (see here).
You could make a stored procedure that performs both the update in table A and the insert in table B
CREATE PROCEDURE proc_name
#id
#param1
AS
BEGIN
update tableA
set field1 = #param1
where ID = #id
insert into tableB(field1)
values(#param1)
END
I have a SQL Server 2008 database. There are three terminals connected to it (A, B, C). There is a table SampleTable in the database, which reacts to any terminal activity. Every time there is some activity on any terminal, logged on to this DB, the new row is inserted into SampleTable.
I want to redirect traffic from one (C) of the three terminals to write to table RealTable and not SampleTable, but I have to do this on DB layer since services that write terminal activity to DB are in Black Box.
I already have some triggers working on SampleTable with the redirecting logic, but the problem is that rows are still being inserted into SampleTable.
What is the cleanest solution for this. I am certain that deleting rows in an inserting trigger is bad, bad, bad.
Please help.
Edit:
Our current logic is something like this (this is pseudo code):
ALTER TRIGGER DiffByTerminal
ON SampleTable
AFTER INSERT
AS
DECLARE #ActionCode VARCHAR(3),
#ActionTime DATETIME,
#TerminalId INT
SELECT #ActionCode = ins.ActionCode,
#ActionTime = ins.ActionTime,
#TerminalId = ins.TerminalId
FROM inserted ins
IF(#TerminalId = 'C')
BEGIN
INSERT INTO RealTable
(
...
)
VALUES
(
#ActionCode,
#ActionTime,
#TerminalId
)
END
In order to "intercept" something before a row gets inserted into a table, you need an INSTEAD OF trigger, not an AFTER trigger. So you can drop your existing trigger (which also included flawed logic that assumed all inserts would be single-row) and create this INSTEAD OF trigger instead:
DROP TRIGGER DiffByTerminal;
GO
CREATE TRIGGER dbo.DiffByTerminal
ON dbo.SampleTable
INSTEAD OF INSERT
AS
BEGIN
SET NOCOUNT ON;
INSERT dbo.RealTable(...) SELECT ActionCode, ActionTime, TerminalID
FROM inserted
WHERE TerminalID = 'C';
INSERT dbo.SampleTable(...) SELECT ActionCode, ActionTime, TerminalID
FROM inserted
WHERE TerminalID <> 'C';
END
GO
This will handle single-row inserts and multi-row inserts consisting of (a) only C (b) only non-C and (c) a mix.
One of the easiest solution for you is INSTEAD OF trigger. Simply stating, it's trigger that "fires" on very action you decide and lets you "override" the default behavior of the action.
You can override the INSERT, DELETE and UPDATE statements for specific table/view (you use it a lot with views that combine data from different tables and you want make the view insert-able) using INSTEAD OF trigger, where you can put your logic. inside the trigger you can then call again to INSERT when it's appropriate, and you don't have to worry about recursion - INSTEAD OF triggers won't apply on statements from inside the trigger code itself.
Enjoy.
I work using SQL Server. I have 2 tables, Animal and Cat. When I add a new cat to the database, I want to update both tables. I should add the cat to the Animal table first, so that I can add the animal_Id to the Cat table afterwards.
Is there a way to add the record at two tables at the same time? If there isn't, what is the best way to do it?
I just want an idea.
If you use a transaction, both inserts will be done, at least logically, "at the same time".
That means that no other query, done from outside of the transaction, can see the base "between the inserts". And if there is a failure between both inserts (and no effective commit), the final state will ignore first insert.
In order to get the id of a row just added in your session, use SCOPE_IDENTITY.
You can't use INSERT against two tables in one statement.
SET XACT_ABORT ON
BEGIN TRANSACTION
INSERT INTO [A](...) VALUES(...);
INSERT INTO [B](...) VALUES(...);
COMMIT TRANSACTION
SET XACT_ABORT OFF
The transaction is to make sure it is everything or nothing is committed. The XACT_ABORT ensures that if one fails with an error (therefore COMMIT TRANSACTION will not fire), the transaction will be forced to roll back.
I would suggest to use transaction here. For example (if you know the Id of new row beforehand):
DECLARE #CAT TABLE(id int, name varchar(50));
DECLARE #ANIMAL TABLE(id int);
DECLARE #anmalId INT = 1;
BEGIN TRAN
INSERT INTO #ANIMAL VALUES(#anmalId);
INSERT INTO #CAT VALUES(#anmalId, 'Kitty');
COMMIT TRAN
SELECT * FROM #CAT;
SELECT * FROM #ANIMAL;
You can use ##identity in case of auto increments.
Use triggers. That is the best way
how about using trigger while insertion to one table??