SQL FOREIGN KEY instance tutorial

SQL FOREIGN KEY instance tutorial

Key constraints of foreign SQL statements
The foreign key is located at one table point, and the primary key is seated at another table point.

Let's explain the foreign key and an example. Take a look at the two tables below:

In the "persons" table:

LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Order table.

O_Id OrderNo P_Id
1 77895 3
2 44678 3
3 22456 2
4 24562 1

Note that the "order" table in the "P_Id" column indicates that "persons" are seated in the "P_Id" column.

The "person" table in the "P_Id" column is the primary key to taking a seat at "person.

The Order table of the foreign key of the Order table in the "P_Id" column.

The foreign key constraint is used to prevent actions from damaging the contact table between them.

Foreign key constraints can also prevent invalid data from being inserted into foreign key columns because it is a value table.

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Foreign SQL key constraints: Table Creation
When the following SQL statement creates the "P_Id" column on the foreign key, the "order" table is created:

MySQL:

Create table Orders
(
O_Id int not null,
OrderNo int not null,
P_Id int,
Primary key (O_Id ),
Foreign key (P_Id) REFERENCES Persons (P_Id)
)
Web SQL Server/Oracle/MS Access
Create table Orders
(
O_Id int not null primary key,
OrderNo int not null,
P_Id int foreign key references Persons (P_Id)
) To get the naming foreign key constraints and determine the foreign key constraints for multiple columns, use the following SQL Syntax: MySQL/SQL Server/Oracle/MS Access: to reduce foreign key constraints, use the following SQL: MySQL: ALTER TABLE Orders
Drop foreign key fk_PerOrdersSQL Server/Oracle/MS Access: alter table Orders
Drop constraint fk_PerOrders