What do you mean by referential integrity take example to explain?
Referential integrity It means the reference from a row in one table to another table must be valid. Examples of referential integrity constraint in the Customer/Order database of the Company: Customer(CustID, CustName) Order(OrderID, CustID, OrderDate)
Can a primary key be a foreign key at the same time?
A Foreign Key is used for referential integrity, to make sure that a value exists in another table. The Foreign key needs to reference the primary key in another table. If you want to have a foreign key that is also unique, you could make a FK constraint and add a unique index/constraint to that same field.
Can a key be both primary and foreign?
Yes, foreign key has to be primary key of parent table. Yes, it may not be unique and may have duplicate entries in child table, but it must be unique and does not have any duplicate entries at the parent table (as it is a primary key).
What is the difference between referential integrity and entity integrity?
Referential integrity is the state in which all values of all foreign keys are valid. Referential integrity is based on entity integrity . Entity integrity requires that each entity have a unique key.
Why foreign keys are not redundant?
Essentially, primary and foreign keys are used as a way to constrain or link related data in a database. This ensures that data remains consistent and that the database contains no redundant data. For example, if you delete a table (or even a row in a table) that other tables rely on, the redundant data is removed.
Does foreign key improve query performance?
It’s a common mistake to avoid creating foreign keys in a database because they negatively impact the performance. It is true that foreign keys will impact INSERT, UPDATE and DELETE statements because they are data checking, but they improve the overall performance of a database.
Do foreign keys always reference Primary Keys?
Creating a foreign key is almost as easy as creating a primary key, except that SQL Server imposes several more rules on foreign keys. For example, the foreign key must reference a primary key or unique constraint, although that reference can be on the same table or on a different table.
Why do we need cross join in SQL?
The CROSS JOIN is used to generate a paired combination of each row of the first table with each row of the second table. This join type is also known as cartesian join.