What is the difference between Primary Key and Foreign Key in SQL?

Question

SQL course

Answer 1

Primary key is used as a unique record identity in a table, primary key usually also be indexed, to support quick search based on the column used as a primary key. A primary key does not depend on any other key, be it in the same table, or a different one.

A foreign key is commonly used as reference to point to another column in different table, to create a "relationship" between tables and to enforce data integrity. A foreign key is also used for table connectors, a connector table contains only primary keys of the tables that it is connecting. A connector table is used for a "many to many" relationship between two tables.

Hth, cmiiw

Answer 2

The Primary Key is one and only in every table. It is unique in the table and always indexed (usually clustered). Every row has a single and unique value as a primary key. You can have a table without primary key but that is an exception and not a good practice as it makes difficult to handle data manipulation for the table.

On the other hand, foreign Keys are fields that are Primary keys on other tables, and commonly are used to create relationship between these two tables giving options (like cascade delete, Insert) to the database administrator.

Answer 3

Think about a primary key like something that identifies the entity among other entities of the same type.

Like the id card, a person has one, never repeats, and it's used to identify the person.

Now if we have cars, someone has to own those cars, might be the dealership, maybe your own car. So let's say, if a car has an owner, it has a foreign key from his owner, so that we know whose car is.

Another example:

table persons
id: 0 name: Mattew
id: 1 name: Sasha

table cars
id_owner: 1 color: red

id_owner is the foreign key and links the tables cars and persons in a way that if we have a car, we can identify who he is, because that foreign key, is primary key in 'persons'

Answer 4

It will be wider topic to explain it here as it include more and more terminology of sql as we go deeper in it.

I would try to explain it in simple words :

Take example of university.

Lets think there may be student table and course table (many more table also.....)

Now enrollment number will be the primary key for student table as it uniquely identity each student and it doesn't have any duplication.

And for course table , course_id will be the primary key.

Now course table will also contain enrollment number attribute or field which is primary key for student table.

Therefore enrollment number is foreign key for course table.

I hope my little example would help you understand it☺️☺️.

Answer 5

u make the primary first because it like parents and the foreign key next because it like child

Answer 6

primary key is unique identity in the table. foreign key is field in the table that in another table is primary key.

Answer 7

There's a good answer to the question here:

https://www.essentialsql.com/what-is-the-difference-between-a-primary-key-and-a-foreign-key/

Answer 8

Las claves primarias se usan para identificar cada tabla de tu base de datos y las claves foraneas se usan para enlazar estas claves primarias desde tablas diferentes.

Answer 9

Thank you very much 😀

Answer 10

Primary key uniquely identify a record in the table.
Foreign key is a field in the table that is primary key in another table.
Primary Key can't accept null values.
Foreign key can accept multiple null value.
By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
We can have only one Primary key in a table.
We can have more than one foreign key in a table.

https://www.dotnettricks.com/learn/sqlserver/difference-between-primary-key-and-foreign-key

Answer 11

Primary Key
- uniquely identify a record in the table.
- can't accept null values.
- By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
- you can have only one Primary key in a table.

*******************************
Foreign Key
- is a field in the table that is primary key in another table.
- can accept multiple null value.
- do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
- you can have more than one foreign key in a table.

Answer 12

primary key have unique value which is never repeat while table is existing in your database

foreign key is use for connecting two or more tables

like if you have one table customer that table should have one primary key which is ID and other table is products which connects with customers table. This is possible with primary key and foreign key

Answer 13

- Primary key: is a field (column) in a database table that uniquly identifies records(rows in the table)
- Foriegn key: is a column or collection of columns that refer to a primary key in another database table.

thanks!

Answer 14

Thank you po and God bless😀

Answer 15

Foreign keys are used to relate to the primary key in another table in order to create a relationship. Let's say you have a table called people where at index 1 is John and he has green eyes and at index 2 is Jane and she has blue eyes. Then you have a second table called 'people with blue eyes' and you put Jane at index 1 but you give her a foreign key of 2 to relate to her place on the 'people' table.