SQL Tutorial 18: SQL foreign key constraint.
Topic covered in this tutorial
1. Defining SQL foreign key using create table at column level
2. Defining SQL foreign key using create table at table level
1. Defining SQL foreign key using Alter Table statement
Website article: http://www.rebellionrider.com/sql-foreign-key.htm
Primary key tutorial: http://www.rebellionrider.com/sql-foreign-key.htm
Create Table Tutorial: http://www.rebellionrider.com/sql-create-table.html
Tool used in this tutorial is SQL Developer.
This tutorial series is part of SQL expert exam certification training. if you are preparing for SQL certification you can use my tutorials. This SQL Tutorial is a part of free training.
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This is Manish from RebellionRider.com
Foreign key is an Input/output data constraint which is also known as referential integrity constraint.
Foreign key represent a link or say a relationship between columns of tables.
Similar to primary key constraint Foreign Key constraint is also of two types.
Simple Foreign key constraint and
Composite Foreign key constraint.
Constraint which involves only one column in foreign key in child table and one column in reference key in parent table is called Simple Foreign Key. While the constraint which involves more than one column in foreign key in child table and more than one column in reference key in the parent table is called Composite Foreign Key.
God I try to make a reference between 2 tables 2 columns for 2 days and I couldn't do it.... I hate your accent but sir.... you are a GREAT TEACHER, thank you in my country they say > Hope you reach 100 y old! or more ... have a great day
can someone please help with this sql i want the user to update his profile. i create a profile page now i cant get the sql correct for the upadte button. i tried
update user (table)
set firstname,lastname,address,email,telephone (fields in my table)
where 'email' (email it uses to update that particular user account details
hi sir, your lectures very informative for me
meny 2 table create kiye hain or aik new 3rd table create kr k just usme un dono tables ki primary keys as a foreign keys lgani hai
kya ap mjhe 3rd tables ka syntax send kr de gy yahan please
Dear Manish, If I have to create more than one primary key, I have to use composite Primary key. Fine. When I try to keep one of this Primary key to my foreign key error is raised . can you give your reply please
You are an awesome Teacher! A lot of things that I never understood, it took me only 10 Min. with you and now I have understand everything! Thanks a lot Bro! Do not stop teaching the World how to do this! =) Saludos!
You got an amazing talent of teaching others , keep on doing this sacred work Dear....
The thing is that I followed your tutorial for installing the oracle 11g complete setup all was well but in the end whenever I login into my Enterprise Manager account it says something like this(INTERNAL ERROR HAS OCCOURED AND DO SOMETHING WITH YOUR LOG FILES)... please tell me I am desperate to see the dashboard of EM opening on my screen....
With Love from PAKISTAN <3
Mr Sharma, I like your videos. I'm using ORACLE SQL 11G express and I have one request? would you continue on with your (this) video and make a video with real database tables to show how FOREIGN KEY work with real data??? thank you in advance!
I have a question.
What if I have table A, table B and table C. Let's say table C is linked with both table A and B. How would I define foreign keys in table C? Could I add two constrains for each reference? Thanks. :)
I know how to define a foreign key - there's a lot of information about this on the Internet. But I do not quite understand the details of how the foreign key is used. THAT would be a very valuable information in your tutorial
+Anatolii Stepaniuk. Imagine you are an IT guy in a company. IT department has ID 5. This means that in "Employees" table there would be "Anatolii" with department_ID=5. Now I go to the separate "Departments" Table and try to delete the department with ID 5. If there is NO Foreign Key, I will successfully delete the department, and "Anatolii" will end up with NO department assinged. And that's bad :). Hope this helped.
Foreign key is a referential integrity constraint which helps in several manners like establishing a relationship (link) between parent and child table which you can then use in several ways for example, Joins is one of them. Uses of FK constraint include:
· 1. It forestalls those actions that can lead to
destruction of links between various tables.
2. It ensures that the data that is being inserted in the
foreign key column is essentially coming from the table it points to. This guarantees
the validity of the data.
Incidentally Primary key has the job of identifying
each row uniquely in a table.
You can go through these links for more knowledge on Primary and Foreign key.
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