foreign key

1Schema::table('posts', function (Blueprint $table) {
2    $table->unsignedBigInteger('user_id');
3
4    $table->foreign('user_id')->references('id')->on('users');
5});
6OR
7Schema::table('posts', function (Blueprint $table) {
8    $table->foreignId('user_id')->constrained();
9});

1ALTER TABLE table_name
2DROP CONSTRAINT fk_name;

1CREATE TABLE Employee(
2    EmployeeID int NOT NULL,
3    LastName varchar(50) NOT NULL,
4    FirstName varchar(20) NOT NULL,
5    Age int,
6    DeptNo int,
7    PRIMARY KEY (EmployeeID),
8    FOREIGN KEY (DeptNo) REFERENCES Department(DeptNo)
9);
10

1Foreign Key: 
2It is a column that comes from a different table and
3using Foreign key tables are related each other
4It is the primary key of another table
5It can be duplicate or null for another table
6
7
8Primary Key :
9It is unique column in every table in a database
10It can ONLY accept;
11    - nonduplicate values
12    - cannot be NULL
13
14Unique Key:
15Only unique value and also can contain NULL

1USE Organization
2CREATE TABLE Employee
3(
4Id INT PRIMARY KEY IDENTITY(1,1),
5Name VARCHAR (50) NOT NULL,
6Age INT,
7Gender VARCHAR (50),
8Dep_Id int FOREIGN KEY REFERENCES Department(Id),
9Insur_Id int FOREIGN KEY REFERENCES Insurance(Id)
10)

1USE Organization
2CREATE TABLE Employee_Office
3(
4Id INT PRIMARY KEY IDENTITY(1,1),
5Emp_Id int FOREIGN KEY REFERENCES Employee(Id),
6Office_Id int FOREIGN KEY REFERENCES Office(Id)
7)