I have 10 tables created in Snowflake. How do i model them? I mean how do i establish relationship between the tables in the form of a parent-child relationship or in the form of Facts and Dimensions?
Can you please share a sample model if you have?
Snowflake supports defining and maintaining below constraints, however it does not enforce them, except for NOT NULL constraints, which are always enforced.
UNIQUE
PRIMARY KEY
FOREIGN KEY
NOT NULL
For constraints and related details please refer
https://docs.snowflake.com/en/sql-reference/constraints-overview.html
For sample code/model you can check at https://docs.snowflake.com/en/user-guide/table-considerations.html
Related
I want to create a many-to-one relationship between my customer and booking tables, however when I enforce referential integrity, only one-to-one shows.
I have several other tables that have worked correctly, so this seems to be a bug; can anyone offer any suggestions?
Summary:
Customer ID is the unique primary key in the Customer table
Customer ID is a foreign key in the booking table
Access is enforcing a one-one relationship between the two
How can I create this many-to-one relationship?
Thank you to everyone who responded, it turned out I had accidentally set the field indexing property to NO Duplicates.
I have two entities - Request and Report. Each Request can have only one Report and each Report can belong to only one Request. This is one-to-one relationship.
Right now I'm thinking about underlying database schema. Right now in table requests I have FK to PK reports.id.
Is it a correct approach ? Where FK to other entity must be placed - in reports or requests table ?
That's all you need regarding the foreign keys. You may want to also add a unique constraint (unique index) to the foreign key for better performance and consistency guarantees.
As mentioned in the title, is it possible to create many-to-many relationship between two tables that belong to two different databases? If yes, how can i perform that with PostgreSQL?
The standard way of using foreign key constraints to enforce referential integrity is only possible within the same database - not db cluster. But you can operate across multiple schemas in the same database.
Other than that, you can create tables just the same way. And even join tables dynamically among remote databases using dblink or FDW. Referential integrity cannot be guaranteed across databases by the RDBMS, though.
Does not matter much whether the other DB is on the same physical machine or even in the same DB cluster - that just makes the connection faster and more secure.
Or you can replicate data to a common database and add standard constraints there.
It should be possible, but as has been stated you cannot expect much in the way of referential integrity.
If you follow the standard design pattern of using a linking table, you can generate a sort of M2M relationship.
DB1.dbo.Users has the USER_ID primary key
DB2.dbo.Tasks has the TASK_ID primary key
you could create a table on either DB1 or DB2 that is UsersToTasks
DB1.dbo.UsersToTasks
USER_ID - KEY
TASK_ID - KEY
This way, a unique pairing of USER_IDs and TASK_IDs are used as a key in that table. The only thing is you cannot create a foreign key to the other table.
As a pseudo workaround, you could write a trigger on DB2.dbo.Task that would write the TASK_ID to DB1.dbo.TASK_IDS and link that as the foreign key on the linking table above. I'm not sure, but you could also potentially create a delete trigger that would remove the TASK_ID as well.
http://solaimurugan.blogspot.com/2010/08/cross-database-triggers-in-postgresql.html
Hi I've set up two very basic tables. One table will act as a look up, with an identity field as a primary key. The other table uses the look up ID as a foreign key.
I have created a relationship constraint so now I cannot delete from the look up if the foreign key is used in the "main" table.
However my issue is i can add a record with a foreign key that doesn't exist.
To my way of thinking this shouldn't be allowed, can anyone tell me what setting I need to use to enforce this and whether this is typical database design or not?
Thanks Dave
You way of thinking is correct. Good database design provides some way of enforcing what is called "Referential Integrity". This is simply a buzzword for the concept you have derived on your own. Namely that a foreign key should be rejected if it refers to a non existent row. For a general discussion of referential integrity, see the following Wikipedia article. It's short.
http://en.wikipedia.org/wiki/Referential_integrity
Some pprogrammers would like to enforce referential integrity inside their programs. In general, it's a much better plan to define a referential integrity constraint inside the database, and let the DBMS do the enforcement. It's easier, it's faster, and it's more effective.
The SQL Data Definition Language (DDL) provides a way to declare a foreign key constraint when you create a table. The syntax differs a little between different dialects of SQL, but it's basically the same idea in all of them. Here's a capsule summary.
http://www.w3schools.com/sql/sql_foreignkey.asp
The documentation for SQL Server should have a description of the referential integrity constraint under the CREATE TABLE command.
If I want to implement the relationship between Category and Classified, is a database-level nullable foreign key required or is it possible/advisable for an application to define this type of relationship without using a database constraint?
[Note: Because the white dot indicates "optional" and the black dot "required", for each Category a Corresponding classified may or may not exist. In addition, the crows feet between them indicate this is a many to many relationship.]
Since it's a many-to-many relationship, you'll want a cross-reference table rather than a simple foreign key column.
So the Category table does not have a FK to Classified, and Classified does not have a FK to Category. Instead you can have a new table :
XrefCategoryClassified
FK to Category NOT NULL
FK to Classified NOT NULL
This is a typical way to implement a many-to-many relationship. And now, instead of worrying about NULLable fields if two records aren't related, you simply care about the existence or non-existence of a xref record
Why not use both?
Foreign keys, check constraints etc are known as "Declarative Referential Integrity" for a reason. They protect your data. What if you add a bulk load next month, or you have to run a SQL script to change data?
Another point would be that the database engine is the correct tool for this.
Absent compelling reasons to do otherwise, I'd enforce referential integrity at the database level; after all that's (partly) what an RDBS is good for.
And since you'll likely have some sort of mapping table to define the many-to-many relationship between Category and Classified, it seems like a no-brainer to put your constraints there. Your queries will thank you for it later.