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mysql foreign key

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Hao-Ting Tai answered on March 2, 2020 Popularity 10/10 Helpfulness 3/10

answer mysql foreign key

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mysql foreign key

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-- ADD foreign key
ALTER TABLE TableName ADD FOREIGN KEY (Table_ID) REFERENCES OtherTable (OtherTable_ID);

Popularity 10/10 Helpfulness 3/10 Language sql

Source: ithelp.ithome.com.tw

Tags: key mysql sql

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Contributed on Jan 07 2023

Hao-Ting Tai

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add foreign key to existing table mysql

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ALTER TABLE contacts
ADD FOREIGN KEY (user_id)
REFERENCES users(id);

Popularity 7/10 Helpfulness 6/10 Language sql

Source: Grepper

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Contributed on Jul 15 2024

@CodeGrepperManu

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foreign key mySql

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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;

Popularity 10/10 Helpfulness 5/10 Language sql

Source: dev.mysql.com

Tags: key

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Contributed on Mar 02 2020

Kaotik

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Mysql Create table with foreign keys.

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CREATE TABLE products(
    productId INT AUTO_INCREMENT PRIMARY KEY,
    productName varchar(100) not null,
    categoryId INT NOT NULL,
    CONSTRAINT fk_category
    FOREIGN KEY (categoryId) 
    REFERENCES categories(categoryId)
        ON UPDATE CASCADE
        ON DELETE CASCADE
) ENGINE=INNODB;

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Source: www.mysqltutorial.org

Tags: create-table create-tab

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Contributed on Apr 06 2022

Henry Obiaraije

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how to connect database foreign key in mysql

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     CREATE TABLE products(
    productId INT AUTO_INCREMENT PRIMARY KEY,
    productName varchar(100) not null,
    categoryId INT NOT NULL,
    CONSTRAINT fk_category
    FOREIGN KEY (categoryId) 
    REFERENCES categories(categoryId)
        ON UPDATE CASCADE
        ON DELETE CASCADE
) ENGINE=INNODB;

Popularity 10/10 Helpfulness 4/10 Language sql

Source: www.mysqltutorial.org

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Contributed on Aug 20 2020

Kelly

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mysql foreign key constraint

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ALTER TABLE `tabl`
  ADD CONSTRAINT `constraint_name` FOREIGN KEY (`column`) REFERENCES `table2` (`id`) ON DELETE CASCADE;

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Source: Grepper

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Contributed on Aug 30 2022

Clumsy Crane

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MySQL Foreign Key

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Here is the basic syntax of defining a foreign key constraint in the CREATE TABLE or ALTER TABLE statement:

[CONSTRAINT constraint_name]
FOREIGN KEY [foreign_key_name] (column_name, ...)
REFERENCES parent_table(colunm_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]
In this syntax:

First, specify the name of foreign key constraint that you want to create after the CONSTRAINT keyword. If you omit the constraint name, MySQL automatically generates a name for the foreign key constraint.

Second, specify a list of comma-separated foreign key columns after the FOREIGN KEY keywords. The foreign key name is also optional and is generated automatically if you skip it.

Third, specify the parent table followed by a list of comma-separated columns to which the foreign key columns reference.

Finally, specify how foreign key maintains the referential integrity between the child and parent tables by using the ON DELETE and ON UPDATE clauses.  The reference_option determines action which MySQL will take when values in the parent key columns are deleted (ON DELETE) or updated (ON UPDATE).

MySQL has five reference options: CASCADE, SET NULL, NO ACTION, RESTRICT, and SET DEFAULT.

CASCADE: if a row from the parent table is deleted or updated, the values of the matching rows in the child table automatically deleted or updated.
SET NULL:  if a row from the parent table is deleted or updated, the values of the foreign key column (or columns) in the child table are set to NULL.
RESTRICT:  if a row from the parent table has a matching row in the child table, MySQL rejects deleting or updating rows in the parent table.
NO ACTION: is the same as RESTRICT.
SET DEFAULT: is recognized by the MySQL parser. However, this action is rejected by both InnoDB and NDB tables.
In fact, MySQL fully supports three actions: RESTRICT, CASCADE and SET NULL.

If you don’t specify the ON DELETE and ON UPDATE clause, the default action is RESTRICT.

MySQL FOREIGN KEY examples
Let’s create a new database called fkdemo for the demonstration.

CREATE DATABASE fkdemo;

USE fkdemo;
RESTRICT & NO ACTION actions
Inside the fkdemo database, create two tables categories and products:

CREATE TABLE categories(
    categoryId INT AUTO_INCREMENT PRIMARY KEY,
    categoryName VARCHAR(100) NOT NULL
) ENGINE=INNODB;

CREATE TABLE products(
    productId INT AUTO_INCREMENT PRIMARY KEY,
    productName varchar(100) not null,
    categoryId INT,
    CONSTRAINT fk_category
    FOREIGN KEY (categoryId) 
        REFERENCES categories(categoryId)
) ENGINE=INNODB;
The categoryId in the products table is the foreign key column that refers to the categoryId column in the  categories table.

Because we don’t specify any ON UPDATE and ON DELETE clauses, the default action is RESTRICT for both update and delete operation.

Popularity 10/10 Helpfulness 2/10 Language sql

Source: www.mysqltutorial.org

Tags: key

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Contributed on Oct 19 2020

Dev

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mysql foreign key

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