While validation can prevent most SQL errors, there are situations that simply cannot be prevented. I can think of two of them: uniqueness of some column and wrong foreign key: validation cannot be effective as the an object can be created or deleted by other parties just after validation and before db insertion. So there are (at least) two SQL errors that should lead to a message of invalid user input.
SQLException has a Number property for the error type, but I don't know how to find out which column is duplicated or which foreign key is wrong without trying to parse the actual error message text, which happens to be localized.
Is there any way to identify the offending column other than parsing the error message (which means at least to strictly choose a language for SQL Server and always use it)?
edit:
I should mention that I come from RubyOnRails, where the approach is: let's pretend that the db doesn't exist: no constraints, no db-enforced foreign keys etc. As I'm approaching ASP.NET MVC, I'd like to get rid of the rails biases, and accept the fact that the db indeed exists.
Are you sure these two situations absolutely cannot be prevented?
You can avoid unique constraint SQLexceptions on Insert, by using an Identity (database generated) primary key column. SQL Server will guarantee that the value is unique.
The same goes for inserting related rows into tables linked by a foreign key. Insert a row in each referenced table first, before inserting a row in the main table. Use IDENTITY_INSERT to get the value of each auto generated primary key and use this as the foreign key in your main table.
You should also wrap these individual statements in a transaction to ensure that either all tables are inserted successfully or none are. The transaction also isolates (hides) these changes from all other concurrent database accesses until the transaction is committed.
Related
before inserting in the table i am validating if the ID already exists in table.
It does not exists. Even though it through this error. Is there any possibility that the table does not allow to insert may be permission issue.
I have that insert query in a procedure, is there a possibility that due to procedure permission it doesn't allow.
If you check in the first statement/transaction for existences and you run this SP in parallel it is possible that the second (INSERT) statement/transaction fails.
In this case you can use a MERGE statement which either inserts the data or updates on existences in one transaction.
Simple thing. While inserting data to table A I have a HashKey id and additional hash index for column ex_id, which is kind of a foreign key in table B.
When inserting a new data into table A I would like to create an exception whenever data is inserted with value in column ex_id that doesn't have a correspondent entry in table B.
I thought that ConditionExpression is the way to go, but can't make it work - probably missing something obvious. Tried to use contains()...
Any ideas?
As per my knowledge this would not be possible at DynamoDB end because there are no relationship between the tables.
What you can do is that you can have a condition at the application level, which checks on its own and throw an exception before inserting the value in table A. (You can query table B for that "Id" if found then insert else throw exception)
DynamoDB does not natively support any kind of foreign key support, everything works on a per table basis, per key basis. DynamoDB's approach is to handle such logic at the client level. For example see the dynamodb transactions client. This library allows you to perform transactions across tables which either all succeed or all rollback.
For your case, I would first make a getItem request to table B (use consistent read) if it exists then write to table A.
Then I would enable streams on table A and write a lambda function to check if any data violations get written to the table.
I was working with LINQ in my asp.net application. I've putted a Unique Key on two columns of the 'ColorDV' table. When trying to add a duplicated value like 'ss' I've got the result:
System.Data.UpdateException: An error occurred while updating the
entries. See the inner exception for details. --->
System.Data.SqlClient.SqlException: Violation of UNIQUE KEY constraint
'TITLE_AR_ColorDV'. Cannot insert duplicate key in object
'dbo.ColorDV'. The duplicate key value is (ss). The statement has been
terminated.
but the problem is that after this exception i can't insert any record in any of the tables, because of the same exact exception. I wonder if LINQ is trying to perform the same previous insert each time.
Please how can I solve this problem?
When you add an object to the ObjectContext, that object stays within the list of objects being submitted to the database, so yes that is the problem. Remove the object and continue, or if an edit try to use the Refresh method on the context to refresh the contents of the object to the original state.
This has an additional approach; can't personally say it works though.
I have an asp.net Gridview that handles insert operations into a SQL database. Records are only permitted to be inserted if they meet a uniqueness criteria, and this constraint is being enforced using unique indexes in SQL server. If the user attempts to insert a record that already exists, an error message is displayed.
I'm wondering what the best practice is for implementing this.
Check if the record exists SQL side, using IF EXISTS, and locking hints (updlock, holdlock, etc). Return an error code to ASP.net depending on whether the record was inserted
Perform the INSERT operation inside a SQL server try/catch block, relying on the unique index to prevent the insert from occurring if the record exists. Return an error code depending on whether an exception was thrown.
Perform the INSERT operation SQL side, but without SQL try/catch. Handle the PK violation exception inside ASP.net instead.
Normally I'd consider using exceptions to handle valid operations to be bad practice - i.e. software should not throw exceptions unless something is broken. However if the unique index on the table in SQL is going to implement the desired constraint, why bother performing a manual check for existence of the record?
I would make a separate call to check if the record already exists. If yes, show message to user, if no make insert. The reason I would do it this way is because I prefer keeping all the business logic in the application.
If you insist in making just one stored proc call:
I would check before I insert. I would also add an output parameter to the stored proc that returns a message if the insert was unsuccessful. In my application if I see a message in the output parameter, I will display that to the user.
Let's say I have two schemas: HR and Orders.
[HR].Employees [Orders].Entries
-------------- ----------------
Id_Employee ----> Employee
Fullname Id_Entry
Birthday Description
Amount
As you can see, what I'd want is to be able to establish a cross-database foreign key, but when I try this using a database link, I get:
-- From [Orders]
ALTER TABLE Entries
ADD CONSTRAINT FK_Entries_Employees FOREIGN KEY (Employee)
REFERENCES Employees#HR;
COMMIT;
ORA-02021: DDL operations are not allowed on a remote database
Is there a way around this? It's a legacy database, so I can't change the existing schema.
For the NHibernate crowd: I would then use this relation to map the NHibernate's domain objects.
One option would be to create a materialized view of Employees on [Orders] and then use that as the parent for the foreign key.
Of course, that has some drawbacks. In particular,
-- you won't be able to do a complete refresh of the materialized view without disabling the foreign key, so it'll have to fast refresh.
-- keys entered into EMPLOYEES won't be available to ENTRIES until the materialized view refresh. If that's critical, you may want to set it to refresh on commit.
Other alternatives are to handle the key enforcement yourself through a trigger or through a post cleanup process. Or convince the DBA's that these schemas can reside on the same database instance.
As far as I know constraints and referential integrity are only supported within one single database.
If you need to cross the boundaries of the database, you'd have to be creative. Maybe write some triggers checking for data in the other database or enforce these constraints on the application level. But then you may encounter the problem with transaction scope limited to one single database.