Forem: Alaa Mkbs

I Was Confused About Merise for Weeks. Here's Everything I Learned

Alaa Mkbs — Sun, 22 Mar 2026 19:33:29 +0000

When I started learning database design, my teacher kept saying "MCD, MLD, MPD" and I had no idea what any of that meant. I searched in English and found almost nothing. That's when I realized Merise is a French methodology, and most of the internet doesn't talk about it.

So I learned it the hard way. This is the guide I wish existed when I started.

What is Merise?

Merise is a French software and database design methodology created in the 1970s-80s. It's widely used in France and French-speaking countries, but almost unknown in the English-speaking world where people use ERD or UML instead.

The core idea of Merise is that you design your database in 3 levels, going from abstract to concrete:

Level	French Name	English Equivalent
MCD	Modèle Conceptuel de Données	Conceptual Data Model
MLD	Modèle Logique de Données	Logical Data Model
MPD	Modèle Physique de Données	Physical Data Model

Think of it like building a house:

MCD = the architect's sketch (ideas, no technical details)
MLD = the blueprint (structure, measurements)
MPD = the actual construction (specific materials, real tools)

Level 1 MCD (Modèle Conceptuel de Données)

MCD is a graphical representation that describes the data of a system and their relationships in an abstract way. You're not thinking about tables or code yet just what exists and how things relate.

Structure:

Rectangles = Entities (contain attributes describing their properties)
Ovals = Relationships/associations between entities (connected to entities with lines)
Cardinalities = Indicate the minimum and maximum number of relations between entities
Attributes = The properties inside each entity

Cardinality the hardest part:

Cardinality answers: "how many of X can be related to Y?"

The format is min,max on each side:

1,1 → exactly one (mandatory, unique)
0,1 → zero or one (optional, max one)
1,N → at least one, can be many
0,N → optional, can be many
N,N → many to many → becomes a junction table in MLD

Example:

1,N next to School → one school has many courses. 1,1 next to Course → one course belongs to exactly one school.

The N-N rule:

In MCD, N-N relationships stay as just an oval. You don't create a table for them yet that happens in MLD.

Level 2 MLD (Modèle Logique de Données)

MLD is the translation of the MCD into a model adapted to relational databases defining tables, columns, primary keys, foreign keys, and relationships between tables.

Structure:

Tables = Come from entities and associations of the MCD
Columns = Based on the attributes from the MCD
Primary Key (PK) = One or more unique attributes identifying each row
Foreign Key (FK) = Links one table to another
No data types yet, no indexes that's MPD

3 rules for converting MCD → MLD:

1) Many-to-One (N:1):
The primary key from the N side becomes a foreign key in the other table.

2) Many-to-Many (N:N):
A new junction table is created. It contains at minimum two foreign keys pointing to both entities. If the association has its own attributes, they go in this table too.

3) One-to-One (1:1):
The primary key of one entity becomes a foreign key in the other table.

Example the N-N becomes a real table:

In MCD you had:

In MLD this becomes 3 tables:

Teacher (id, name, specialty)
Class (id, name)
TeacherClass (id, teacher_id FK, class_id FK)

TeacherClass didn't exist in MCD as a table it was just an oval. Now it's a real table. (we can add some attributes if needed)

Level 3 MPD (Modèle Physique de Données)

MPD = MLD + everything specific to your database engine.

You add:

Data types (VARCHAR(255), INT, TIMESTAMP, BOOLEAN…)
Constraints (NOT NULL, UNIQUE, DEFAULT…)
Indexes
Engine-specific options (PostgreSQL vs MySQL vs SQLite)

Example:

MLD says:

User (id, email, password, role)

MPD says:

CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  email VARCHAR(255) NOT NULL UNIQUE,
  password TEXT NOT NULL,
  role VARCHAR(20) NOT NULL DEFAULT 'STUDENT',
  created_at TIMESTAMP(6) DEFAULT NOW()
);
CREATE INDEX idx_users_email ON users(email);

If you use an ORM like Prisma, your Prisma schema is your MPD because it has types (@db.VarChar(255)), indexes (@@index), and database-specific decorators.

Why Is There No English Documentation?

Honestly, this frustrated me a lot. Every time I searched "MCD MLD database design" in English, I got nothing useful.

The reason is simple: Merise is French. It was created by French researchers, taught in French schools, and used by French companies. English-speaking developers grew up with ERD (Entity-Relationship Diagrams) and UML, which are different tools that do roughly the same thing.

It's not that Merise is bad it's actually very structured and clean. It's just that the internet is mostly in English, and English devs never learned it.

If this helped you, drop a comment. And if you know better Merise resources in English, please share them we need more.

The difference between for...of and for...in loops in JavaScript

Alaa Mkbs — Thu, 04 Dec 2025 22:59:36 +0000

When working with JavaScript, understanding the difference between for...of and for...in loops is crucial for writing clean, efficient code. While they might look similar, they serve different purposes and work with different data structures.

With Arrays:

for...of gives you the actual elements
for...in gives you the indices (as strings)

const data = ['apple', 'banana', 'cherry'];

for (let item of data) {
    console.log(item);
}
// Output:
// apple
// banana  
// cherry

const data = ['apple', 'banana', 'cherry'];

for (let index in data) {
    console.log(index);
}
// Output:
// 0
// 1
// 2

With Strings:

for...of gives you each char in the word
for...in gives you the indices (as strings)

const text = "hello";

for (let char of text) {
    console.log(char); // h, e, l, l, o
}

for (let index in text) {
    console.log(index); // 0, 1, 2, 3, 4
}

With Objects:

for ... of doesn't work directly with plain objects (This would throw an error)
for ... in gives you key value for each element

const person = { name: 'John', age: 30, city: 'NYC' };

// for...in works with objects
for (let key in person) {
    console.log(key, person[key]);
}
// Output: name John, age 30, city NYC

How to Reset a Forgotten MariaDB User Password

Alaa Mkbs — Sat, 14 Dec 2024 15:48:15 +0000

Did you or someone using MariaDB forget the password to an account? Don’t worry! Resetting a MariaDB user password on Linux is super easy. Here’s a quick guide with the commands and step-by-step instructions to help you out.

Change MariaDB password using command line

Open a terminal.
Stop the currently running MariaDB database.

sudo systemctl stop mariadb

Restart the database process, but this time use the --skip-grant-tables option. This lets you connect to the database without needing a password.

sudo mysqld_safe --skip-grant-tables --skip-networking

Now, open a new terminal.
Login to MariaDB as root. Don't worry, it won't ask you about the password.

mariadb -u root

Run the FLUSH PRIVILEGES command and then change the root password with the command below. Change this NEW_PASSWORD_HERE to your new password. Past these commands line by line inside MariaDB Monitor.

FLUSH PRIVILEGES;
ALTER USER 'root'@'localhost' IDENTIFIED BY 'NEW_PASSWORD_HERE';
exit

Close your first terminal.
Stop the current mysqld process properly, and then restart your MariaDB server. Line by line.

sudo pkill mysqld 
sudo systemctl start mariadb

Congratulations! You’ve successfully changed the password. Now, go ahead and test it out!

mariadb -u root -p

Thanks for reading!