What is the Difference Between Primary Key and Foreign Key

KEY DIFFERENCES: A primary key constrain is a column that uniquely identifies every row in the table of the relational database management system, while foreign key is a column that creates a relationship between two tables.

1. What is difference between primary and foreign key?
2. What is the difference between primary key and foreign key with example?
3. What is primary key and foreign key in SQL?
4. What is an example of a foreign key?
5. Is foreign key always a primary key?
6. Can foreign key be null?
7. Why is a primary key important?
8. What is primary key and example?
9. Can a table have both primary key and foreign key?
10. Can Unique Key be a primary key?
11. What data type is a primary key?
12. Why foreign keys are not redundant?

What is difference between primary and foreign key?

Primary key is used to identify data uniquely therefore two rows can't have the same primary key. It can't be null. On the other hand, foreign key is used to maintain relationship between two tables. Primary of a table act as forgein key in the other table.

What is the difference between primary key and foreign key with example?

A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table. ... Only one primary key is allowed in a table.

What is primary key and foreign key in SQL?

Difference between primary key and foreign key in SQL:

Primary key uniquely identify a record in a table while foreign key is a field in a table that is primary key in another table. There is only one primary key in the table on the other hand we can have more than one foreign key in the table.

What is an example of a foreign key?

Definition: Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables. For example: In the below example the Stu_Id column in Course_enrollment table is a foreign key as it points to the primary key of the Student table.

Is foreign key always a primary key?

If you really want to create a foreign key to a non-primary key, it MUST be a column that has a unique constraint on it. ... A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.

Can foreign key be null?

Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). ... Null by definition is not a value.

Why is a primary key important?

Choosing a primary key is one of the most important steps in good database design. A primary key is a table column that serves a special purpose. Each database table needs a primary key because it ensures row-level accessibility. ... The values that compose a primary key column are unique; no two values are the same.

What is primary key and example?

A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence. For example, students are routinely assigned unique identification (ID) numbers, and all adults receive government-assigned and uniquely-identifiable Social Security numbers.

Can a table have both primary key and foreign key?

Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.

Can Unique Key be a primary key?

A primary key must be unique. A unique key does not have to be the primary key - see candidate key. That is, there may be more than one combination of columns on a table that can uniquely identify a row - only one of these can be selected as the primary key.

What data type is a primary key?

A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have only one primary key, which may consist of single or multiple fields.

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.