How can I add a constraint that references a foreign column from another database?
Some time ago I read that it can be done with linked server and others say with triggers. What's the preferred way of doing this if that's possible at all?
Thanks!
Linked servers will not work.
FK's must point to local tables.
The preferred way is not to do this, though you can pull a few hacks to make it happen.
For example you could have triggers cause the reference to be created/checked but I wouldn't consider that equivalent to a FK constraint.
Related question:
Can you have a Foreign Key onto a View of a Linked Server table in SQLServer 2k5?
Use an instead of trigger (you may or may not need linked servers depending on whether the other database is on a differnt server). Make sure the trigger can handle multiple row inserts/updates/deletes. I'd also suggest moving the records that fail the check to an exception table.
Foreign keys cannot go across database boundaries. If you try to do this, you'll get:
Msg 1763, Level 16, State 0, Line 1
Cross-database foreign key
references are not supported. Foreign key ***
Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.
If you need to enforce some kind of relationship between two separate databases, then yes - you might need a linked server (if that second database is on a second server), and possibly triggers - but all of those things will be very hard to get right, very inefficient and very error prone.
One way you might be able to do this would be data replication - replicate the table you want to reference into your source database, and then establish a foreign key relationship with that replicated table. But that will never be quite "live" and "real-time" - there will also be a bit of a lag in the data replication.
Related
I have a database that I built a while back. Every table in the database is InnoDb. Several tables had foreign key constraints, and I set them up for On Delete = Cascade. When I was using an earlier version of phpmyadmin, working with these was simple: I'd just go to the Structure tab of a table, click the Relation View link, and as long as I had the correct indexes set up on the correct columns, I could set the foreign keys as I saw fit.
Since upgrading to version 4, it's become a nightmare. For some tables, I go to the relation view and everything is just fine. But for others--even when they already have foreign key constraints set--I can't see any options for working with them.
To make matters worse, I've even tried dropping the indexes and re-adding them, only to be given the following error: Cannot drop index [index_name]: needed in a foreign key constraint. So unless I'm mistaken, the constraint is there, but phpmyadmin is refusing to show it to me.
Is there something I have to do to make them show up again? This is extremely frustrating to say the least: something that worked just fine before now does not thanks to an upgrade.
OK, after playing around with the tables a bit, I figured out what's going on. The only time the foreign key constraint options don't show up are when the table names contain capital letters. Very frustrating to say the least.
I just filed a bug report for phpmyadmin: https://github.com/phpmyadmin/phpmyadmin/issues/11461
It should be an easy fix.
happened to me because i used '&" in the database name.
In my case it is that I used two columns (A and B) both as foreigns keys to other tables then I also used a composite unique for ([A, B]), phpMyAdmin does not show the existed foreign index of column A but does show that for column B.
My system version are as follows:
Server version: 5.7.30 - MySQL Community Server (GPL)
I have created one database by executing generated script(schema).
After that I am copying database,only data while copying I facing problem of foreign key violation and primary key violation
What should I do for this issue.
key Violation
You really only have two options here, go though the data your importing and change the conflicting value or drop the constrains all together. Once the data is in the new DB you can not enable constraints until you address the duplicate data. See this SO for in on removing FKs, it also applies to dropping PKs
How do I drop a foreign key in SQL Server?
I would highly recommend seeing if you can change the data so it no longer violates the key constrains
I am trying to restore a DB using an SQL script, but things foreign key constraints get in the way
I am taking a MySQL DB and bringing it over to PostgreSQL.
Since the MySQL create table syntax ended up being quite different, I took another PostgreSQL DB with the same schema, but different data and restored the schema only, from that.
In other words, I now have a database with tables, constraints, sequences and all that shnaz but no data inside.
So, it's is time to restore data.
I take a backup of the MySQL DB with phpMyAdmin (data only) as an SQL script (pgAdmin does not seem to accept zip or gzip files for some reason) and run the SQL script.
Now, this is where the problems start to happen, it's only natural, I am going from MySQL to PostgreSQL, so syntax errors are bound to happen.
But, there are other non syntax related problems to, like this one:
ERROR: insert or update on table "_account" violates foreign key constraint "fk_1_account"
DETAIL: Key (accountid)=(2) is not present in table "_entity".
So, yeah, basically, a foreign constraint exists, the query is trying to insert data into the _account table, but the corresponding data has not been inserted into the _entity table yet.
How do I get around that? Is there a way to make pgAdmin3/PostgreSQL disable ALL OF the constraints, insert the data, and then re-enable the constraints?
A syntax related error I encountered, was this one:
INSERT INTO _accounttype_seq (id) VALUES (11);
The PostgreSQL equivalent of that statement (if I am correct) is
ALTER SEQUENCE _accounttype_seq INCREMENT BY 11;
But, it's a bit of a pain to run through the whole script and change all 200+ Sequence insert statements. So, I am being lazy here, but is there an easier way to deal with the sequences as well?
Or, do you guys have any suggestions for a different set of tools to make this easier?
Thanks for your time, have a good day.
Do not try to get around the foreign key constraints. That is the way to make sure the data is bad.
First look at the constraints and make sure you are inserting to the tables in the correct order. If _entity is parent of "_account, then it should be populated first.
Next you need to have the script move any failing records to an exception table. Then you can look at them and see what the data integrity issues is and if you need to throw the records away permanently or try to figure out what the missing parent value should be. If it is critical data such as orders where the customer no longer exists (possible in any system that didn't have correct fks to begin with) and you must keep the record and cannot determine what the parent value should have been, you can create an 'Unknown" record in the customer table and assign all bad orders to that customer id.
And manually changing the alter sequences shouldn't take long even if it is boring. There wil be plently of other things you need to handle manually in a conversion of this type.
I would try to find a data import tool for PostgreSQL - I live in SQL server world where I would use SSIS but you need the equivalent of SSIS for the PostgreSQL world.
Aparently the foreign keys weren't actually enforced in MySQL (maybe because of using MyISAM) or the generated SQL just does it in the wrong order.
If it's "only" the wrong order, I see two possible solutions:
edit the generated script and either move all FK definitions to the end of the script
Edit the definition of each FK constraint and set them all to initially deferred. Then run the script as one single transaction with only on commit at the very end.
Edit (because this is too much to be put as a comment)
Using SET CONSTRAINTS ALL DEFERRED will only work if the constraints have been created with the option DEFERRABLE.
To run everything in one single transaction, you have to make sure you have turned autocommit off. Then simply run the INSERTs and at the very end issue a COMMIT. A ; will only commit if you have autocommit on.
If you want to be independent of the autocommit setting, then start your script with [BEGIN][1] and make sure there is only a single COMMIT at the very end.
BEGIN DEFERRABLE
INSERT INTO table_one ... ;
INSERT INTO table_two ... ;
.....
COMMIT;
I'm writing some code to inspect a MySQL database structure, and need information about Foreign Key constraints (on InnoDB tables).
There are two ways I know of to do this:
Parse the results of SHOW CREATE TABLE X
Use INFORMATION_SCEMA.REFERENTIAL_CONSTRAINTS
Unfortunately option two requires MySQL 5.1.16 or later, so I can't use it unless/until I can convince our server guy to update, And while I can probably get away with option 1, it feels messy and without writing a full SQL parser I wouldn't feel sure my code would always work with any table.
Is there another way of getting at this information?
Thanks
From the MySQL 5.0 manual online:
You can also display the foreign key constraints for a table like
this:
SHOW TABLE STATUS FROM db_name LIKE 'tbl_name';
The foreign key constraints are listed in the Comment column of the
output.
Poster indicates that this doesn't provide ON UPDATE and ON DELETE information which is an important part of foreign key behavior.
Another option:
Since you control the code involved, is it possible to set up another MySQL instance in the same environment which is version 5.1+? If so, let's call that instance dummy. Run the SHOW CREATE TABLE on the live database. Then, on dummy run a DROP TABLE IF EXIST followed by the output from the SHOW CREATE TABLE query.
Now you can use INFORMATION_SCHEMA on the dummy database to get the information.
three servers
server 1 is central point which links the other two servers.
the other two servers have a table which has a field which should act like a foreign key
problem
well i do not know how to do this, using vs08 or sql server 08
diagram view
table on server 1
sv1pg1 id -- primary key
details
tables same schema on srvr2 and srvr3
linkedSRVid -- pk
linkedto -- fk constrain should be between (server 1 primary key and this field)
note that
"this is just a simplest! way i can think of to represent the prob, the real dbs contain
many fields and tables and the following applies"
it's not always that the network connection between the server remains up.
the link only need for "writing purpose" not viewing, as viewing done by direct
connection to the respective server.
A foreign key constraint can reference other tables only within the same database. This means that even if those databases were on the same server it still would not work.
Considering that the schemas are the same, you may want to look into replication.