Please help me out.

So I just started a project and I would be using PHP and MySQL and Python for this project and then I came across the need of foreign keys which I don't really understand much.

Please can someone explain foreign keys and show me a way I can use them.

Thanks.

Foreign are the primary key of related tables.

Let's say a Users table and Posts tables has a one to many relationship i.e. A user can have one or more posts. In the post table, you'll have these columns.

Posts
id - primary key
user_id //this refers to the id on the user.
text
pictures

In the users table, you'll have.

Users
id - primary key
name


The work of foreign key is to reference that a particular user is the owner of certain post(s).

When you insert values into the posts, it'll be in this format:

Posts
id - 1 //the ID of the post also the primary
//in posts table
user_id - 1 //this is the primary key on users
// table which refers to the user of
//id 1, but in posts table here it is
// called the foreign key.
text - "SQL is SQL"
pictures - "/pictures/sql.jpg"


So now, a user has a post.

There's a bit more to this though.

Karleb:
Foreign are the primary key of related tables.

Let's say a Users table and Posts tables has a one to many relationship i.e. A user can have one or more posts. In the post table, you'll have these columns.

Posts
id - primary key
user_id //this refers to the id on the user.
text
pictures

In the users table, you'll have.

Users
id - primary key
name


The work of foreign key is to reference that a particular user is the owner of certain post(s).

When you insert values into the posts, it'll be in this format:

Posts
id - 1 //the ID of the post also the primary
//in posts table
user_id - 1 //this is the primary key on users
// table which refers to the user of
//id 1, but in posts table here it is
// called the foreign key.
text - "SQL is SQL"
pictures - "/pictures/sql.jpg"


So now, a user has a post.

There's a bit more to this though.

Thanks a lot for your help

Karleb:
Foreign are the primary key of related tables.

Let's say a Users table and Posts tables has a one to many relationship i.e. A user can have one or more posts. In the post table, you'll have these columns.

Posts
id - primary key
user_id //this refers to the id on the user.
text
pictures

In the users table, you'll have.

Users
id - primary key
name


The work of foreign key is to reference that a particular user is the owner of certain post(s).

When you insert values into the posts, it'll be in this format:

Posts
id - 1 //the ID of the post also the primary
//in posts table
user_id - 1 //this is the primary key on users
// table which refers to the user of
//id 1, but in posts table here it is
// called the foreign key.
text - "SQL is SQL"
pictures - "/pictures/sql.jpg"


So now, a user has a post.

There's a bit more to this though.

Is there an website where I can visit to understand more or maybe a YouTube video

Karleb:
Foreign are the primary key of related tables.

Let's say a Users table and Posts tables has a one to many relationship i.e. A user can have one or more posts. In the post table, you'll have these columns.

Posts
id - primary key
user_id //this refers to the id on the user.
text
pictures

In the users table, you'll have.

Users
id - primary key
name


The work of foreign key is to reference that a particular user is the owner of certain post(s).

When you insert values into the posts, it'll be in this format:

Posts
id - 1 //the ID of the post also the primary
//in posts table
user_id - 1 //this is the primary key on users
// table which refers to the user of
//id 1, but in posts table here it is
// called the foreign key.
text - "SQL is SQL"
pictures - "/pictures/sql.jpg"


So now, a user has a post.

There's a bit more to this though.

this your explanation is quite confusing.
A primary key is unique key
while a foreign key is not a unique key. A foreign key of one table is used to reference a primary of another table when there is reletionship between this table.
This kind of relationship is called one to many relationship.
for example, taking from Data analysis perspective, if we have two tables, customers table and sales table. The customers id column in the customers table shows the unique customers that make purchase everyday, this column has a unique entries (no entry appears twice) and it's called the primary of the costumers table.
now in the sales table we might have a column called customer id or any other thing ,not necessarily costumer id, with entries that are not unique, which can be used to match up the customer id column of the customers table, this column is called the foreign key.
The foreign key is not unique. Its entries appear many times in a column.
For you to better understand it well, just imagine that we have 3 customers in our online store database, and we give them customer id of 1,2,3. This customer id will be unique in the customers table but in the sales table of the customers id this numbers will appaer many times since a customer makes transaction many times, customer 3 might make 4 transactions in a day and in the costumers Id column it appear like this 3,3,3,3. so are

CenturyCoder:


Is there an website where I can visit to understand more or maybe a YouTube video

Personally, I prefer www.w3schools.com because their explanations is always to the point, but then you can always Google search any question you don't understand.

But I believe the best way to learn is to practice.

Model Model relationships such a has one, has many, belongs To Many will demystify everything.

Its Matching one Model to another Model thats the easiest explanation i build Django apps with MySQL

It's very simple.

Take English Premier League for example

Currently we have 20 clubs in the league, and each club plays a certain number of games.

Now, if you are to represent this in an Application, you will have at least 2 tables as follows:

1. Fixtures
2. Results

Now, ordinarily, you will want to attach each club details to their fixtures and results. But this will lead to a lot of data repetition across the database.

So,to eliminate this data repetition,we can create a third table named
club_details

Now we can put all the club data in this Club details table, and give each club a Unique numeric ID. This ID is normally called the Primary Key, and it's usually Auto Increment.

So, our club_details table can have the following fields:
Id
club_name
stadium


So, in the Fixtures and Results table now, we don't need to insert the club details again. All we need to do is just use the ID from club_details table.

So, our fixtures table have the following fields

game_Id (primary key)

home_team_id (refers to the Id of the home team from club details)

away_team_id (refers to the Id of the away team from club details)

Since both home_team_id and away_team_id refer to the Id in club_details table, both of them can be set as Foreign Keys.


So, this is just a basic example of Foreign Keys.

---

But, in the real sense, you don't really need foreign keys for your beginner project.

The only major advantage of Foreign Keys is that it keeps your database consistent, by performing checks at the database level before inserting, updating or deleting data.

So, by setting Foreign Keys, before inserting a new data in the Fixtures table, the database will check if the ID exists in the club_details table. If the ID does not exist, the insert fails.

Meanwhile, if you don't use foreign keys, it will just perform the insert without checking, even if the ID does not exist.

This also applies to Updating and Deleting data.

Personally, I don't use Foreign Keys in my projects except I'm working with a framework, as most frameworks (like Laravel and Django) also use it for Data relationships.

I just want to say thank you to all who contributed to this page, thanks a lot

.