INU Login Sign up
Login Or Sign up

Answers for "what is a foreign key"

SQL
25

mysql add foreign key

-- On Create
CREATE TABLE tableName (
    ID INT,
    SomeEntityID INT,
    PRIMARY KEY (ID),
    FOREIGN KEY (SomeEntityID)
        REFERENCES SomeEntityTable(ID)
        ON DELETE CASCADE
);

-- On Alter, if the column already exists but has no FK
ALTER TABLE
  tableName
ADD
  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
  
 -- Add FK with a specific name
 -- On Alter, if the column already exists but has no FK
ALTER TABLE
  tableName
ADD CONSTRAINT fk_name
  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
Posted by: Guest on April-27-2020
19

foreign key in sql

A FOREIGN KEY is a key used to link two tables together.
A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.

Example:
# creating table users
CREATE TABLE users(
	user_id INT NOT NULL,
  	user_name VARCHAR(64) NOT NULL,
  	user_pass VARCHAR(32) NOT NULL,
  	PRIMARY KEY(user_id);
);
# adding user data
INSERT INTO users VALUES(1,"Raj","raj@123");

# creating table orders
CREATE TABLE orders(
	order_id INT NOT NULL,
  	order_description VARCHAR(255),
  	orderer_id INT NOT NULL,
  	PRIMARY KEY(order_id),
  	FOREIGN KEY (orderer_id) REFERENCES users(user_id)
);
# adding order data
INSERT INTO orders VALUES(1,"Daily groceries",1);
Posted by: Guest on April-25-2020
1

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)
);
Posted by: Guest on November-01-2021
0

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)
);
Posted by: Guest on July-06-2021
4

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 NULL
Posted by: Guest on January-27-2021
0

foreign key constraint

CREATE TABLE Students ( 	 /* Create table with foreign key - Way 1 */
    ID INT NOT NULL
    Name VARCHAR(255)
    LibraryID INT
    PRIMARY KEY (ID)
    FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID)
);

CREATE TABLE Students ( 	 /* Create table with foreign key - Way 2 */
    ID INT NOT NULL PRIMARY KEY
    Name VARCHAR(255)
    LibraryID INT FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID)
);


ALTER TABLE Students 	 /* Add a new foreign key */
ADD FOREIGN KEY (LibraryID)
REFERENCES Library (LibraryID);
Posted by: Guest on July-06-2021

Code answers related to "what is a foreign key"

Code answers related to "SQL"

Browse Popular Code Answers by Language

Python
Javascript
Whatever
Shell/Bash
CSS
Html
PHP
SQL
Java
C#

Popular Programming Languages

Abap ActionScript Assembly BASIC C C# C++ Clojure Cobol CSS Dart Delphi Elixir Erlang F# Fortran Go Groovy Haskell Html Java Javascript Julia Kotlin Lisp Lua Matlab Objective-C Pascal Perl PHP Prolog Python R Ruby Rust Scala Scheme Shell/Bash SQL Swift TypeScript VBA Whatever

Advertisements

290x500

Company

Compilers

Help

Connect with us

footer logo

Copyright © 2021 Codeinu