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.
Related
i want to know if it's possible to drop a table fragment that is not letting me perform a restore with the NDB_RESTORE tool.
When i run the restore, it throws the following error:
Create table db_died_maestro/def/NDB$FKM_3194_0_mae_tipo_reg_evaluacion failed: 721: Schema object with given name already exists
Restore: Failed to restore table: db_died_maestro/def/NDB$FKM_3194_0_mae_tipo_reg_evaluacion ... Exiting
NDBT_ProgramExit: 1 - Failed
I have already drop the DB_DIED_MAESTRO database previous to run the restore, but this fragment is not being dropped along with the database.
I have check that the fragment is in the database catalog using this querys:
*select * from ndbinfo.operations_per_fragment
where fq_name like 'db_died_maestro%'*
query result
And this query:
*select * from ndbinfo.memory_per_fragment
where fq_name like '%FKM_3194_0_mae_tipo_reg_evaluacion'*
query 2 result
This fragment was created on a previous run of the NDB_RESTORE tool. Please help me.
The table is a foreign key 'mock' table (indicated by the name NDB$FKM prefix).
Foreign key mock tables are created transiently in some cases to implement the foreign_key_checks = 0 feature of MySQL. This feature requires storage engines to support unordered creation of tables with partially defined foreign key constraints which can be abritrarily enabled (without revalidation) at a later time.
Foreign key mock tables are normally entirely managed by the Ndb storage engine component of MySQL, and so should not be visible unless there has been a failure or bug of some kind.
If you can share information about activities occurring before this problem then that would help us understand how this happened and whether it can be avoided.
As a workaround it should be possible for you to use the ndb_drop_table utility to drop this table, before re-attempting the failing restore. You may have to escape the $ in the name passed as a command line argument from a shell. Probably you should check for any other NDB$FKM tables in a similar situation.
I am trying to run some basic CREATE TABLE statements for my Databases course project and am getting some strange errors.
When I create the table Manuf it runs fine, but when I try to create the next table, Order, using the same syntax, it does not work.
Also, when I try to create this table, Items, I get an errno: 150. I believe this has to do with my foreign key creation, but I am not exactly sure. Here is a screenshot of that.
I am fairly new to using MySQL so any advice would be greatly appreciated, thank you.
The error on the Order table is caused by ORDER being a reserved word. You can specify it as `Order` with the backticks, but it's better if you choose a different name altogether.
The error 150 is related to the foreign key. The keys must be absolutely identical - the exact same definition, or the FK will fail with error 150.
Also, there must be an available index with that key definition or one compatible (see Kai Baku's example in the comment on the MySQL manual page). The same fields indexed in a different order will fail.
To begin with, check how those keys are defined in the origin tables. For example:
test1 varchar(50) not null
test2 varchar(50)
will not be compatible. I think that even a different collation is enough to throw FK off kilter (but this I haven't checked. The rest I'm sure of, from my personal bitter unexperience).
UPDATE: I forgot to mention, if you use InnoDB tables and issue the SHOW ENGINE INNODB STATUS, the blurb that comes out will contain a much better explanation of why the FK failed, somewhere about one third from top.
In MySQL, using MySQL Workbench, how can you create a foreign key relationship to a table which resides in another database? After entering the 'Qualified Table Name', of DatabaseName.TableName.ColumnName, workbench simply acts like I put nothing in the box. Do I need to create these relationships only with manual query in workbench if it is a different database?
InnoDB engine allows you to define FK(Foreign Key) for tables.(even from other databases)
For this you must mention database name before table and column name. Like so :
dbname.tablename.columnname
I think I found the solution. Actually you have to add another schema in your Model which represents your 'different' database.
Then you need to create all your tables (in an EER Diagram) into this new Schema.
Finally, when you'll click on the Foreign Keys tab from the original table you'll be able to choose the right table from the different database in Referenced Table. For example different.new_table
I think it is impossible to do what you want. You can not link tables from different databases.
I'm trying to create some meta-data to extend mysql functionality but I can't create tables in the database INFORMATION_SCHEMA. I thought that I could just create another database and have my metadata in there but I need some foreign keys from my tables to some tables in the INFORMATION_SCHEMA DB. Nevertheless, I get errors when trying to create them. Then I thought I could create a trigger to get notified of changes but since triggers are associated to a table and I can't alter that database, I can't create triggers either.
Specifically I have some tables that references to information_schema.schemata(schema_name) and to information_schema.schemata(columns) and some others. I want to have those foreign key so I can use ON UPDATE CASCADE ON DELETE CASCADE or otherwise I'll have some rows in my tables referencing to nothing and I can't allow that.
I'm using mariaDB 5.5.30 which uses MySql 5.3.
INFORMATION_SCHEMA tables are actually views whose contents is automatically maintained by the MySQL server.
The manual gives more information:
Inside INFORMATION_SCHEMA there are several read-only tables. They
are actually views, not base tables, so there are no files associated
with them, and you cannot set triggers on them. Also, there is no
database directory with that name.
Although you can select INFORMATION_SCHEMA as the default database
with a USE statement, you can only read the contents of tables, not
perform INSERT, UPDATE, or DELETE operations on them.
They are not really views but temporary tables, that's why you don't see folders.
show create view views;
ERROR 1347 (HY000): 'information_schema./tmp/#sql_2ac_0' is not VIEW
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;