Is it possible to enforce a FK constraint with only JPA annotations and not in the DB?
I want to Restrict/No Action on delete to enforce that a FK reference cannot be deleted while it is being used in a parent table. However, I have not been able to accomplish this functionality without adding the FK Constraint at the DB level.
Is this possible or is it required to also add the constraint to the DB as well?
Thank you
Nope, it's not possible. JPA relies on the DB to enforce FK constraints.
With Hibernate, you can provide a custom delete query using #SqlDelete. You could e.g. use a procedure to check if the constraint is going to be violated. I imagine that's not what you're looking for, though.
Related
Is there a way for constraints to actually work in Snowflake?
A primary key is created. Still duplicates can be inserted in the table. Giving options like cascade update and delete cascade are not working with Foreign key
Can someone please help?
if you read the Snowflake documentation you will see that only NOT NULL constraints are enforced, all other constraint types are informational only.
I am guessing that the reason for this is that Snowflake is an analytical, rather than an OLTP, database and therefore the expectation is that constraints are enforced in your ELT processes (as is normal practice) rather than in the DB.
Snowflake does not enforce constraints except not null.
Snowflake Notes . I think we cannot enforce a constraint in snowflake database but you can apply the constraint in your ETL tool(if using)
Hi I've set up two very basic tables. One table will act as a look up, with an identity field as a primary key. The other table uses the look up ID as a foreign key.
I have created a relationship constraint so now I cannot delete from the look up if the foreign key is used in the "main" table.
However my issue is i can add a record with a foreign key that doesn't exist.
To my way of thinking this shouldn't be allowed, can anyone tell me what setting I need to use to enforce this and whether this is typical database design or not?
Thanks Dave
You way of thinking is correct. Good database design provides some way of enforcing what is called "Referential Integrity". This is simply a buzzword for the concept you have derived on your own. Namely that a foreign key should be rejected if it refers to a non existent row. For a general discussion of referential integrity, see the following Wikipedia article. It's short.
http://en.wikipedia.org/wiki/Referential_integrity
Some pprogrammers would like to enforce referential integrity inside their programs. In general, it's a much better plan to define a referential integrity constraint inside the database, and let the DBMS do the enforcement. It's easier, it's faster, and it's more effective.
The SQL Data Definition Language (DDL) provides a way to declare a foreign key constraint when you create a table. The syntax differs a little between different dialects of SQL, but it's basically the same idea in all of them. Here's a capsule summary.
http://www.w3schools.com/sql/sql_foreignkey.asp
The documentation for SQL Server should have a description of the referential integrity constraint under the CREATE TABLE command.
I have created tables in sql server. And i have also inserted data/rows in that tables.
Now i want to make relationship among them means i want to create foreign key constraints among them, is it possible ?
Whenever i try to create relationship among table a problem is occured. "Saving changes is permitted, The changes you made required table to re-created and dropped"
Please suggest me what should i do to make relationship(foreign key) among them ?
My Child table design is this
this is my parent table:-
please now right what alter query i should write..?
You can try this link
"Error message when you try to save a table in SQL Server 2008: "Saving changes is not permitted"
Another solution is below.
I think the problem is because of a feature when using the GUI. If you have a look at this link it shows you how to work round it. It is a feature which prevents you from dropping and recreating the table which is what SSMS does in the background when you click ok.
The code provided by the previous posted is the best way to do this.
You could do this with a script like this:
alter table ChildTable
add constraint FK_ChildTable_ColumnName foreign key (ColumnName) references ParentTable(PrimaryKeyColumnName)
[Edit] If I read your description correctly the script would be:
alter table emp
add constraint FK_emp_salary foreign key(salary) references testing(roll)
You can only add foreign constraints that aren't violated by existing data. You may also have to add suitable indices first, although this depends on the DBMS. In any case, first make sure your existing data is compatible with the foreign keys you want to introduce. In other words, if you were to create the foreign key first, and then insert the data, you should not produce any foreign key violations.
Do i need to setting the foreign key for this situation ?
i'm weak in database design, especially in mysql.. may i know if i want to setting foreign keys for them, what should i setting for them ? in case if the people delete... all referral to people_id will delete together, is it possible to set while the table is too many ?
Thx for reply
Yes. Foreign key constraints enforce referential integrity, a key tenet of ensuring that your data is reliable and of high quality. Otherwise, your people_address table could reference a people_id value that doesn't exist in the people table, and would be an orphan. A foreign key constraint would prevent that from happening.
So, just do it. There's really no good reason not to.
Define foreign keys such as the following on the people_email table:
ALTER TABLE people_email ADD CONSTRAINT FOREIGN KEY (people_id) REFERENCES people (id) ON DELETE CASCADE;
This will mean that you cannot enter a record in people_email where the people_id in that table does not exist in people. Also, if you delete the parent row in people, the rows referencing it in people_email with get automatically deleted.
I personally prefer to manually delete all the rows from the child tables and not use cascade deletes though. It's a bit of extra app dev work, but it makes me feel safer and also allows me some control over locking and ensuring that queries are as efficient as possible.
I have two tables 'Elements' and 'Lists'
Lists has a primary key and a list name.
Elements has data pertaining to an individual entry in the list.
Elements needs a column that holds which list the element is in.
I've read about SQL's foreign key constraint and figure that is the best way to link the tables, but I'm using SQLite which doesn't enforce the foerign key constraint.
Is there a point to declaring the foreign key constraint if there is no enforcement?
It's always good to do, even if your database doesn't enforce the constraint (old MySQL, for instance). The reasoning for this, is that someday, someone will try reading your schema (perhaps even yourself).
If you can't use the new version, you can still declare the constraint and enforce it with triggers. In either case, I wouldn't omit the notation. It's far too helpful.
Nowadays sqlite enforces foreign keys, download the new release.
A foreign key is a field (or fields)
that points to the primary key of
another table. The purpose of the
foreign key is to ensure referential
integrity of the data. In other words,
only values that are supposed to
appear in the database are permitted.
It only enforces the "business rule". If you require this from the business side, then yes, it is required.
Indexing will not be affected.
You can still create indexes as requred.
Have a look at Foreign Key
and
Wikipedia Foreign key