I was searching about natural keys and found somewhere that they are made from the data that exist outside the current database..I am not getting this ..how a key can be built using outside data...?
If its using foreign key than what is the difference between them.?
Related
This a newbie security/console question...I created a key ring in my project in a specific (wrong) location, Europe.
I can't see any way in the console to edit or even delete a key ring. The key ring is completely empty...no keys in it.
How can I edit/delete a key ring?
Sorry, you can't delete or rename keys or key rings. We were concerned about the security implications of allowing multiple keys or key versions over time to have the same resource name, so we decided to make names immutable. (And you can't delete them, because we wouldn't be able to do a true deletion--there would still have to be a tombstone tracking that this name had been used and couldn't be reused).
We're aware that this can make things untidy, but we have no immediate plans to change this.
If you want to avoid getting billed for a key or otherwise make it unavailable, you can do so by deleting all the key versions; neither keys nor key rings are billed for, just the active key versions within the keys.
Thanks for your question and for using GCP and Cloud KMS!
Interesting. For comparison on AWS keys have unique IDs and there is a separate resource to alias names to ids.
Your question: How can I edit/delete a key ring?
Visit Destroy a key version. You can destroy an enabled or disabled key version. You may also disable and enable the KMS API. I just did it.
So im trying to add an primary key and a foreign key to my table, but i just can't seem to get it to work. i've checked the forums already and other places but it didn't answer my problem. So here is my table:
http://puu.sh/mVW7D/5986e08daa.png
im trying to get VeiederID as my foreing key and keep studentnr as primary
but ive tryied to "alter table" and add the foreing key as constraint, but i might be doing it wrong, im very new to mysql. any help is appirciated
Steffen
Two points:
You do not identify the second table. A foreign key works between two tables and you are only showing one.
You do not specify which mysql version you are using.
Regardless: You should read the manual, reference: v 5.7 Foreign Keys
Update
I see from your updated example you are creating 3 tables and you want the rows in Veileder to reference studentinfo. So, the statement in the Veiled table should be
FOREIGN KEY (VeilederID)
REFERENCES studentinfo(VeiederID)
ON DELETE CASCADE
Apologies if the spelling is wrong. One other note, it appears you are trying to make the primary key (VeilederID) map to the foreign key. If I am wrong please comment but I don't know if that would be allow. Typically a table HAS it's own primary and a reference constrain (a.k.a. FOREIGN KEY) as a different column.
think i found it now i dont get an error while im running it this way, but if its right im not sure..
http://puu.sh/mW1Ze/ae333a40d6.png
I have two tables in MySQL, named manuals and library. Both could have uploaded files, so I have made an third table, named files. In files, I have a column parent_id. Can I make parent_id as foreign key from both manuals and library?
I am using Laravel (4.2) and tried this, but it doesn't work:
$table->integer('parent_id')->unsigned();
$table->foreign('parent_id')->references('id')->on('library');
$table->foreign('parent_id')->references('id')->on('manuals');
No. You cannot have multiple foreign keys on the same column. From the MySQL documentation:
MySQL supports foreign key references between one column and another within a table. (A column cannot have a foreign key reference to itself.) In these cases, “child table records” really refers to dependent records within the same table.
The reason for this is that MySQL won't be able to differentiate between the parents. Laravel (or any other framework) does not provide a work around for this issue.
This question as been asked in slightly different forms before. Example: it is possible to reference one column as multiple foreign keys
First of all, my apologies if this question is a duplicate - but I find it difficult, putting short, precise words on my problem.
I've got these entities.
The left contains several groups (like in Unix, in order to make data available to a whole group at once) and at the moment, it's always 1. The right one contains projects - and the middle one makes sure, that one group can gain access to several projects.
As you can see, there are foreign key relationships among them. Now, I want to create a new project in nmd__tree. When doing that, it returns an error:
Cannot add or update a child row: a foreign key constraint fails
(nmd.nmd__tree, CONSTRAINT FK_nmd__tree FOREIGN KEY (treeid)
REFERENCES nmd__helperusergrouphierarchy (treeidfk))
This makes sense, since the nmd_tree relies on a valid foreign key in the helper entity - but at the same time, it presents the problem, that the treeidfk isn't yet known, since it is autogenerated in nmd__tree
A solution could be to remove the relations, insert the record in nmd__tree, extract the newly written primary key (treeid) and create a record in the middle helper entity with the new id. It will work, but is really not very elegant. Also, removed relations presents other, potential problems.
My intentions are to create a query, that deals with this problem by creating both records at once. I know, it isn't possible to make a double insert and found this suggestion (my version doesn't write any records), as well as an article, suggesting stored procedures, which I don't see should make a difference
I would really appreciate a push in the right direction, please.
It seems you've got your constraints defined in the wrong direction; The middle table should have two foreign key constraints not the two end tables. That way, you can insert records in the two end tables and then link them up using the middle table.
I went over a legacy database and found a couple of foreign keys that reference a column to itself. The referenced column is the primary key column.
ALTER TABLE [SchemaName].[TableName] WITH CHECK ADD
CONSTRAINT [FK_TableName_TableName] FOREIGN KEY([Id])
REFERENCES [SchemaName].[TableName] ([Id])
What is the meaning of it?
ALTER TABLE [SchemaName].[TableName] WITH CHECK ADD
CONSTRAINT [FK_TableName_TableName] FOREIGN KEY([Id])
REFERENCES [SchemaName].[TableName] ([Id])
This foreign key is completely redundant and pointless just delete it. It can never be violated as a row matches itself validating the constraint.
In a hierarchical table the relationship would be between two different columns (e.g. Id and ParentId)
As for why it may have been created quite likely through use of the visual designer if you right click the "Keys" node in object explorer and choose "New Foreign Key" then close the dialogue box without deleting the created foreign key and then make some other changes in the opened table designer and save it will create this sort of redundant constraint.
In some cases this is a preferred way to reduce redundancy in your model. In using the self referencing foreign key (as shown in you example) you create a hierarchical relationship between rows in your table. Pay attention to what happens when you delete a row from the table, cascading on delete might remove rows you still want.
Using these sort of keys moves some of the data validation to the DB model as opposed to making this a responsibility of the program/programmer. Some outfits prefer this way of doing things. I prefer to make sure programs and programmers are responsible - data models can be hard to refactor and upgrade in production environments.