Hey there! Let's break down each of these ER-to-relational mapping scenarios with clear, real-world examples—no overly technical jargon, just practical explanations. I'll cover each case step by step, including whether they're actually used in real systems.

What it means #

First, 完全参与（Total Participation） for the "one" side means every instance of that entity must be part of the relationship—no exceptions. For example, if we have a Department (one) to Employee (many) relationship, total participation on Department means there are no empty departments; every department must have at least one employee.

Mapping Method #

• Create separate tables for both entities.
• Add the primary key of the "one" entity as a foreign key to the "many" entity's table.
• Enforce total participation by adding constraints to ensure every record in the "one" table has at least one matching record in the "many" table (this can be done via database triggers, application-level checks, or in some databases, specialized CHECK constraints).

Real-World Example #

Suppose we're building a company's HR system that doesn't allow empty departments:

-- 强实体表：部门
CREATE TABLE Departments (
    dept_id INT PRIMARY KEY,
    dept_name VARCHAR(50) NOT NULL,
    location VARCHAR(100) NOT NULL
);

-- 多端表：员工，外键关联部门
CREATE TABLE Employees (
    emp_id INT PRIMARY KEY,
    emp_name VARCHAR(50) NOT NULL,
    salary DECIMAL(10,2) NOT NULL,
    dept_id INT NOT NULL, -- 因为部门完全参与，这里外键不能为NULL
    FOREIGN KEY (dept_id) REFERENCES Departments(dept_id)
);

This is super common in small to mid-sized businesses where unused departments are eliminated to cut costs.

What it means #

Total participation on both sides means every instance of each entity must be part of the relationship. For example, Student and Course with an "Enrolls" relationship: every student must take at least one course, and every course must have at least one student enrolled.

Mapping Method #

• Create tables for both entities, plus a junction table to represent the relationship.
• The junction table will contain the primary keys of both entities as foreign keys (both set to NOT NULL).
• Add constraints to ensure every record in the entity tables has at least one matching entry in the junction table.

Real-World Example #

A university's mandatory course system where students can't graduate without taking required courses, and courses are canceled if no one enrolls:

-- 学生表
CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    student_name VARCHAR(50) NOT NULL,
    major VARCHAR(50) NOT NULL
);

-- 课程表
CREATE TABLE Courses (
    course_id INT PRIMARY KEY,
    course_name VARCHAR(100) NOT NULL,
    credits INT NOT NULL
);

-- 中间表：选课关系，带成绩属性
CREATE TABLE Enrollments (
    student_id INT NOT NULL,
    course_id INT NOT NULL,
    grade CHAR(2),
    PRIMARY KEY (student_id, course_id),
    FOREIGN KEY (student_id) REFERENCES Students(student_id),
    FOREIGN KEY (course_id) REFERENCES Courses(course_id)
);

Universities often use this structure to track mandatory enrollment and avoid offering unpopulated courses.

Weak entities are entities that can't exist on their own—they depend on a strong entity. There are two types of relationships with weak entities:

3.1 标识关系（Identifying Relationship） #

What it means #

The weak entity's primary key includes the strong entity's primary key. For example, OrderItem (weak) depends on Order (strong)—an order item can't exist without an order, and its unique identity is tied to the order it belongs to.

Mapping Method #

• Create a table for the strong entity.
• Create a table for the weak entity, using a composite primary key that includes the strong entity's primary key (this acts as both the weak entity's identifier and a foreign key).
• The foreign key in the weak entity table must be NOT NULL (since weak entities have total participation).

Real-World Example #

E-commerce order details:

-- 强实体表：订单
CREATE TABLE Orders (
    order_id INT PRIMARY KEY,
    order_date DATE NOT NULL,
    customer_id INT NOT NULL
);

-- 弱实体表：订单明细，复合主键包含订单ID
CREATE TABLE OrderItems (
    order_id INT NOT NULL,
    item_seq INT NOT NULL, -- 订单内的商品序号
    product_name VARCHAR(100) NOT NULL,
    quantity INT NOT NULL,
    price DECIMAL(10,2) NOT NULL,
    PRIMARY KEY (order_id, item_seq),
    FOREIGN KEY (order_id) REFERENCES Orders(order_id)
);

This is standard in e-commerce systems—you can't have an order item that isn't linked to an order.

3.2 无标识关系（Non-Identifying Relationship） #

What it means #

The weak entity has its own unique primary key, but still depends on a strong entity (it can't exist without it). For example, FamilyMember (weak) depends on Employee (strong)—each family member has a unique ID (like a national ID), but can't be in the system unless they're linked to an employee.

Mapping Method #

• Create a table for the strong entity.
• Create a table for the weak entity with its own primary key, plus a foreign key linking to the strong entity's primary key (set to NOT NULL).

Real-World Example #

Company employee family records:

-- 强实体表：员工
CREATE TABLE Employees (
    emp_id INT PRIMARY KEY,
    emp_name VARCHAR(50) NOT NULL,
    position VARCHAR(50) NOT NULL
);

-- 弱实体表：员工家属，有独立主键
CREATE TABLE FamilyMembers (
    family_member_id VARCHAR(20) PRIMARY KEY, -- 身份证号
    emp_id INT NOT NULL,
    family_name VARCHAR(50) NOT NULL,
    relationship VARCHAR(20) NOT NULL,
    dob DATE NOT NULL,
    FOREIGN KEY (emp_id) REFERENCES Employees(emp_id)
);

Many companies use this to track dependent benefits—family members are only added if they're linked to an employee.

What it means #

A one-to-one relationship where the relationship itself has attributes (not just the entities). For example, Employee and EmployeeFile (each employee has exactly one file, each file belongs to one employee), with a descriptive attribute archive_date (when the file was stored).

Mapping Method #

You have two options, depending on participation:

1. Merge into one table: If both entities have total participation (every employee has a file), combine all attributes into a single table.
2. Separate tables: If participation is optional, or to follow normalization rules, create two tables and store the descriptive attribute in the table representing the dependent entity.

Real-World Example #

Employee file management with archive date:

-- 主表：员工
CREATE TABLE Employees (
    emp_id INT PRIMARY KEY,
    emp_name VARCHAR(50) NOT NULL,
    position VARCHAR(50) NOT NULL
);

-- 关联表：员工档案，带描述性属性归档日期
CREATE TABLE EmployeeFiles (
    emp_id INT PRIMARY KEY, -- 一对一关系，用员工ID作为主键
    file_number VARCHAR(20) NOT NULL,
    archive_date DATE NOT NULL, -- 关系的描述性属性
    FOREIGN KEY (emp_id) REFERENCES Employees(emp_id)
);

This structure keeps employee basic info separate from file-specific details, which is useful for access control (HR might manage files while others manage employee data).

Let's split this into two parts:

• 完全参与性: Yes, absolutely. By definition, a weak entity cannot exist independently—it relies entirely on a strong entity. That means every weak entity instance must participate in the relationship with its strong entity (no orphaned weak entities).
• 多基数: No, not always. While most weak entities have a many-to-one relationship with their strong entity (like multiple order items per order), one-to-one weak entities do exist. For example, an EmployeeBadge—each employee has exactly one badge, and each badge belongs to exactly one employee. The badge is a weak entity (it's useless without an employee) but has a one-to-one cardinality.

In short: Weak entities always have total participation, but their cardinality with the strong entity isn't limited to "many".

内容的提问来源于stack exchange，提问作者Nirmith Akash