I take Trains as one table and passenger as other table.
train table has train_no, train_name.
Passenger table has pnr and train_no.
i would like to create train_no as foreign key in passengers table.
please provide sufficient code for it..
i am rookie in it.
Thanks in advance
Parse is MongoDB like Object DBMS.
Concept of FK constraints is not available in Parse.
In order to use referencing you have to use Pointers. Although you can write some code for having FK constraint satisfaction (One easy option for the same is to use beforeSave feature of Cloud Code.)
Related
My question is related to the ER-diagram I designed using Oracle SQL developer. I designed this ER-diagram but I don't know how to read the relationships between these tables.
I have created this ER diagram:
ER Diagram
As it can be seen that these relations don't look like those normal one to many or many to one relations. Can anyone please help me how to read the relation between SYS.GENERAL_LEDGER_ACCOUNTS and SYS.INVOICE_LINE_ITEMS? Thanks in advance
The relationship is described by the foreign key:
ALTER TABLE invoice_line_items
ADD CONSTRAINT line_item_fk_accounts
FOREIGN KEY (account_number)
REFERENCES general_ledger_accounts(account_number);
Defines a foreign key with the name line_item_fk_accounts on the column account_number which references the account_number of the general_ledger_accounts.
Assuming that it is notNULL then this is a many-to-one relationship such that each row of invoice_line_items has a relationship to exactly one row in the general_ledger_accounts and there can be many invoice_line_items for each general_ledger_accounts.
Similarly, line_items_fk_invoices is a many-to-one constraint with many invoice_line_items each referencing one row of the invoices table.
Aside: NEVER modify the system schemas. If you do you risk making the database unusable and invalidating any support contract that you have with Oracle. Instead, you should create a user and then work in that schema.
I am using talend for ETL I don't have enough experience in this, I am having two tables for example- account and account_roles account table is having id, name, password etc fields and account_roles table is having account_id which is f.k to account table's pk. and one more field.
Both the fields in account_roles are having duplicates, I want to save account_roles in destination with update and insert logic using talend.
But I am getting error as I don't have any table that can be treated as primary key in the account_roles table, so talend can't update or insert it.
How I deal with this situation I tried tDBOutput advance option use_field_option but still it requires unique entries.
Is there any possible solution to this issue, I also want to know if I can make table Foreign key in the account_roles table will it work then? If yes then How to make F.k in talend OPen studio is my second question.
Attaching Snapshots of my tables and tMap below -
I want to know the way I can put my tables into database if I don't have any primary key! Kindly help me.
First question
I think you should place the primary key in the physical account_roles table. Talend will use the key indication of the dbOutput component, and the physical key of the table.
In order to delete duplicates rows, you can also use a tUniqRow before the dbOutput: The key you indicate in the UniqRow is not directly linked to the database; this is only the key on which tUniqRow will be based.
Second question
It's not possible to delegate the f.k. verification to Talend. But you can do this verification in your database by placing foreign keys in your table. If an id is not present in the reference table, the database returns an error that is returned by Talend.
I was trying to create a hive table with a foreign key relationship with another table but I am facing errors with that.
Isn't it possible to implement a foreign key relationship in a hive table?
hive does not implement foreign keys refer to Hadoop Tutorials which describes as below:
Like any other SQL engines, we don't have any primary keys and foreign keys in Hive as hive is not meant to run complex relational queries. It's used to get data in easy and efficient manner. So while designing hive schema, we don't need to bother about selecting unique key etc. Also we don't need to bother about normalizing the set of data for efficiency.
I am wondering if a PL/SQL (oracle) table can carry three foreign keys? thanks in advance if any one can help me in this regard.
There is no explicit limit on the number of foreign keys on a table. However, there is a limit of 1000 columns per table, so that probably constitutes a practical limit.
Here is a SQL Fiddle which creates a toy table with five foreign keys.
There is not limit on foreign keys use except logic which based behind use of foreign keys, and if one table needs too much foreign keys, which is not logic wise, and database design suffers in such scenario.
As well as 1000-column constraint of oracle tables and pl/sql procedures also have limit in code.
I need to insert records into a table that has no primary key using LINQ to SQL. The table is poorly designed; I have NO control over the table structure. The table is comprised of a few varchar fields, a text field, and a timestamp. It is used as an audit trail for other entities.
What is the best way to accomplish the inserts? Could I extend the Linq partial class for this table and add a "fake" key? I'm open to any hack, however kludgey.
LINQ to SQL isn't meant for this task, so don't use it. Just warp the insert into a stored procedure and add the procedure to your data model. If you can't do that, write a normal function with a bit of in-line SQL.
Open your DBML file in the designer, and give the mapping a key, whether your database has one or not. This will solve your problem. Just beware, however, that you can't count on the column being used for identity or anything else if there isn't a genuine key in the database.
I was able to work around this using a composite key.
I had a similar problem with a table containing only two columns: username, role.
This table obviously does not require an identity column. So, I created a composite key with username and role. This enabled me to use LINQ for adding and deleting entries.
You might use the DataContext.ExecuteCommand method to run your own custom insert statement.
Or, you might add a primary key to a column, this will allow the objects to be tracked for inserts/updates/deletes by the datacontext. This will work even if the column isn't really an enforced primary key in the database (how would linq know?). If you're only doing inserts and never re-use a primary key value in the same datacontext, you'll be fine.