foreign key mySql
CREATE TABLE parent (
id INT NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (
id INT,
parent_id INT,
INDEX par_ind (parent_id),
FOREIGN KEY (parent_id)
REFERENCES parent(id)
ON DELETE CASCADE
) ENGINE=INNODB;foreign keys
CREATE TABLE Employee(
EmployeeID int NOT NULL,
LastName varchar(50) NOT NULL,
FirstName varchar(20) NOT NULL,
Age int,
DeptNo int,
PRIMARY KEY (EmployeeID),
FOREIGN KEY (DeptNo) REFERENCES Department(DeptNo)
);foreign key
CREATE TABLE Employee(
EmployeeID int NOT NULL,
LastName varchar(50) NOT NULL,
FirstName varchar(20) NOT NULL,
Age int,
DeptNo int,
PRIMARY KEY (EmployeeID),
FOREIGN KEY (DeptNo) REFERENCES Department(DeptNo)
);foreign key
Foreign Key:
It is a column that comes from a different table and
using Foreign key tables are related each other
It is the primary key of another table
It can be duplicate or null for another table
Primary Key :
It is unique column in every table in a database
It can ONLY accept;
- nonduplicate values
- cannot be NULL
Unique Key:
Only unique value and also can contain NULLCopyright © 2021 Codeinu
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