I found the same definition for referential and relational integrity. Are they the same? I have researched the two terms separately but cannot find any good definitions.
Yes, they are the same thing. The term "Referential Integrity" is used much more though.
Referential Integrity relates to Foreign Keys in a Relational DB. For a table to exhibit Referential Integrity, the Foreign Key must either be null or reference an existing PK value in the related table.
Related
Apologies in advance if this is a silly question. I recently faced this question while revising for databases, so I wish to check if my understanding is correct.
I understand that participation constraint is referring to the number of instances of an entity that can participate in a relationship. (E.g. Total and Partial).
Referential integrity is also a constraint that is related to relationships in a table, stating that a foreign key must reference an existing primary key of another entity.
Based on what I have read, I feel that referential integrity constraint is more about ensuring data integrity with regards to foreign keys, while participation constraint is more about just describing the relationships between two entities.
However, I keep having the feeling that Total Participation and Referential Integrity have a huge similarity. For example, let's say we have two tables
Customer(CustID, CustName)
Order(OrderID, CustID, OrderDate).
As CustID in the Order table must match a valid CustID in the Customer table, there is a referential integrity constraint here. Am I right to say that Order is also totally participating with Customer in a relationship?
Snowflake allows UNIQUE, PRIMARY KEY, FOREIGN KEY and NOT NULL constraints but I read that it enforces only NOT NULL constraint. Then what is the purpose of other keys and under what circumstances do we have to define them? I appreciate any examples.
Thank you,
Prashanth.
They express intent, helping people understand your data models. Data modeling tools can use them to generate diagrams. You can also programmatically access them to validate data integrity yourself.
Constraints
Snowflake supports defining and maintaining constraints, but does not enforce them, except for NOT NULL constraints, which are always enforced.
Constraints are provided primarily for data modeling purposes and compatibility with other databases, as well as to support client tools that utilize constraints. For example, Tableau supports using constraints to perform join culling (join elimination), which can improve the performance of generated queries and cube refresh.
Constraints could also improve the query performance:
Extended Constraint Properties
RELY | NORELY
Specifies whether a constraint in NOVALIDATE mode is taken into account during query rewrite.
By default, this constraint property is set to NORELY.
If you have ensured that the data in the table does comply with the constraints, you can change this to RELY to indicate that the query optimizer should expect the data in the table to adhere to the constraints. Setting this can improve query performance (e.g. by eliminating unnecessary joins).
Understanding How Snowflake Can Eliminate Redundant Joins
In some cases, a join on a key column can refer to tables that are not needed for the join. If your tables have key columns and you are using and enforcing the UNIQUE, PRIMARY KEY, and FOREIGN KEY constraints, Snowflake can improve query performance by eliminating unnecessary joins on key columns.
Eliminating an Unnecessary Left Outer Join
Eliminating an Unnecessary Self-Join
Eliminating an Unnecessary Join on a Primary Key and Foreign Key
The way I understand it, in the context of relational databases, a database has referential integrity if all referenced attribute values exist. In other words, if a value of one attribute references a value of another attribute, then the referenced value must exist. However, I cannot find any clear definition of referential integrity constraints. What exactly is a referential integrity constraint?
When we create relational database tables, we have to use foreign key columns. It is obvious, otherwise we can not create relationships.
However, I noticed that it is enough to have a foreign key column, you do not need to say that there is a foreign key relationship in table A with table B.
As long as you can write the queries you can retrieve the data.
Do we use this concept for make thing easy? I know, when I look at a database table schema which has marked what columns are foreign key columns, it is easy to understand and start to work with it.
Is there any other reasons?
The point is Referential integrity. If you don't enforce it, sooner or later a bug in the code or some other accident happens and your database is left in an inconsistent state. These inconsistencies are very hard or impossible to fix afterwards.
When we create relational database tables, we have to use foreign key
columns. It is obvious, otherwise we can not create relationships.
Incorrect. You do not need to create foreign keys (though it's a good idea), and they do not represent relationships. They enforce the integrity of the relationship. A foreign key makes sure that a value in one column exists in another column.
However, I noticed that it is enough to have a foreign key column, you
do not need to say that there is a foreign key relationship in table A
with table B. As long as you can write the queries you can retrieve the data.
Yes, the relationship is based on the data itself, not by the inclusion of a foreign key. Also, foreign keys do not need to be between two tables, a table can have a foreign key to itself.
Do we use this concept for make thing easy?
No, we use foreign keys to enforce integrity. That they happen to make ERD diagrams easier to understand is simply a bonus.
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.