I'm trying to create a 1:1 relationship at MySQL Workbench (without using Diagram). I have two tables called agricultural_machine and tractor. And I would like a relationship 1:1 being that the agricultural_machine_id is referenced with tractor_id.
Foreign Keys at agricultural_machine table
Foreign Keys at tractor table
The Diagram is being formed like this
I would like this
In order to implement true 1:1 relationships you need deferrable constraints. That means the foreign key contraints in this case need to be ignored for a short time (probably until the end of the transaction) while you insert "the other row" in the other table. Once both rows are inserted the foreign key constraint can be reactivated and validated now. Otherwise, which row do you insert first? Any insert will fail since the FK won't be validated.
Unfortunately MySQL does not implement this standard SQL feature, so you won't be able to do it. You'll need to change the database to PostgreSQL or Oracle to implement 1:1 relationships.
Related
I'm using symfony 4 and when I do links between tables there are indexes created. I do not understand how they work.
What is their purpose and why is it impossible to import data file into those files.
Is it possible to 'bypass' this mysql restriction and import data-files into mysql even if there is a constraint (without breaking the indexes)?
Thanks
MySQL is a relational database system. If there is a relationship between 2 tables, it is there for a reason. Think of your entities in Symfony. If one entity (a primary entity) has several related entities and those related entities can only exist if there is a primary entity, then the constraint must exist to prevent orphaned records.
For example:
Consider the relationship between companies and divisions. A company can have 0 or more divisions, but a division can only exist as a part of a company. In this case, a record in the division table would have a reference to a record in the company table. However, a record in a company table would have no direct reference in MySQL to any records in the division table.
To prevent a division from being created without a company, you cannot insert a row into the division table that does not reference a record in the company table.
To finally answer the question directly, you can only import data that does not reference the primary table by removing the foreign key constraint (this will not destroy the index). Keep in mind that this will likely result in records that are "orphaned" and do not fit the business model you are trying to create.
Rather than removing the foreign key constraint, you should first import the data to the referenced table (scoring?) and then update the data being imported to contain the correct ids to reference the primary table.
Based on the information in the image provided, each record in the file being imported should have a value for scoring_id that is equal to (presumably) the id field in the scoring table. If there is some other piece of data that can be used to link the 2 tables, use that and configure your entities appropriately.
Mysql FOREIGN KEY is used for data integrity. So what happens is you have a Foreign key which refers to column in another table .It's a way to link data relationships between tables
When Using Import if that key does not exist in the other table the mysql import will throw errors .
If you remove the FOREIGN KEY flag you would be able to import that table
refer to MYSQL manual
on Foreign Keys
Foreign keys will check if the other table has already that element that you try to enter.
What you can do for the import temporarily
use FOREIGN_KEY_CHECKS
Before the import run
SET FOREIGN_KEY_CHECKS=0;
and When it is finished
SET FOREIGN_KEY_CHECKS=1;
Or
use DISABLE KEYS:
ALTER TABLE table_name DISABLE KEYS;
and when the import is finished:
ALTER TABLE table_name ENABLE KEYS;
Of course you have to change table_name
I'm using Mysql workbench EER where I draw my ERD.
There are all kind of relationships between the tables (optional/mandatory,non identifying/identifying)
and I use the Forward engineering to generate the underlying SQL.
As far as the relationships go, for an optional relationship it generates a NULL FK
while in a mandatory a NOT NULL FK and that's it.
I mean shouldn't it also based on the relationships generate Cascades deletes for example?
i.e if I have an identifying relationships, then when the parent is deleted the child should be deleted too thus a cascade delete would have been generated
Or, in other words the relationships modeled in an ERD have no practical value other than conceptually know how your database is modeled on paper?
For example what should be the generated sql code for an identifying mandatory relationship?
What you are referring to is called a relationship. In database management terms a relation is something different.
Your relationships should get implemented as foreign keys. The practical value of a foreign key is that it enforces referential integrity. Cascaded deletes are not typically the desired behaviour and in SQL the default referntial integrity action is that delete of a row in a parent table is not permitted if the row is being referenced in another table. If you want cascaded deletes then you have to specify that. Note that in MySQL only the InnoDB database engine supports referential integrity.
What are the pros and cons of creating table relationships in a MySQL database using queries (JOINS) as opposed to doing it with DDL using Foreign key and referential integrity constraints? I have received a database that has not relationships (No FK) on its tables to identify relationships among tables. The relationships are being created on JOINS when data is being queried.
The main reason for using foreign keys is that data is always consistent in the database. This is "independent" from joins - when you use foreign keys you still need to use joins.
In MySQL it also has a nice side effect: if you use foreign keys, you have to define indexes which might speed up queries.
But with foreign keys you can make sure, that the row which you are referring must exist in the database. See https://en.wikipedia.org/wiki/Foreign_key.
Do I have to fill the field of a foreign key in MySQL or any other database manager?.
I'm writing the data of a table and when I get to the field that is a FK from another table, I have to write something, is this necessary?
I understand that the value in that FK is stored inside the parent table where it comes from.
You have to provide a value unless the foreign key column is nullable.
It depends on whether the is actually a foreign key constraint in place (available in InnoDB only). In some cases frameworks, applications, or database management tools create "false" foreign keys that exist only in the application and not actually in the database. Also, the limits on how you can insert/update/delete data realted to the foreign keys can differ based on the type on constraint in place.
Here is the MYSQL documentation for definitive information:
http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
Specifically, look at the "Referential Actions" section for comments on the behavior between the tables.
I'm currently building a database and I'm not sure when to use foreign keys on the tables.
Most of the tables are for transaction, logging, and history.
Is there a general rule in which tables to put the foreign keys?
I have this table for the general details of a specific transaction, then another table for the specific details. Should I put a foreign key on the table for the specific details?
There's also this table used for storing user info and another for logging user activities. Should I also place a foreign key to the table that logs user activities?
If there is a relation between two tables you should always enforce that using a foreign key constraint.
I prevents logically "corrupted" data (e.g. details for a transaction that doesn't actually exist) and - equally important - it documents your database model.