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Adding foreign key to an existing table in MySQL

schedule Aug 10, 2023
Last updated
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MySQL
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We can add a foreign key to an existing table in MySQL using the following general syntax:

ALTER TABLE table_name ADD FOREIGN KEY (column_name) REFERENCES parent_table (parent_column);

Where:

  • table_name is the name of the existing table we would like to add a foreign key to.

  • column_name is the name of the column we would like to make the foreign key.

  • parent_table is the name of the table within which the column we reference is contained.

  • parent_column is the name of the column to link to using the foreign key.

Example

Consider the following two tables:

pupil table:

id

name

1

axel

2

bobby

3

cathy

Refer here for code to generate the pupil table.

product table:

id

name

bought_by

1

computer

1

2

hair dryer

3

Refer here for code to generate the product table.

To add a foreign key constraint to bought_by column of product table to refer to id column of pupil table:

ALTER TABLE product ADD FOREIGN KEY (bought_by) REFERENCES pupil (id);
Query OK, 2 rows affected (0.03 sec)
Records: 2 Duplicates: 0 Warnings: 0

Now we can make sure that any value we have for bought_by has a corresponding matching value in the id column of pupil. This means we will always have information on the name of the pupil who bought a particular product.

robocat
Published by Arthur Yanagisawa
Edited by 0 others
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